To my experience, having fragile vehicle with lower wing loading is preferable over having high-loaded yet sturdy vehicle at reentry. Critical phase of speed shedding comes actually at 6-7 km/s speeds, with still light structural loads but difficult to manage heat loads, even at reduced-slope powered reentry. Reentry vehicle surviving through that speed range would likely survive to the rest of reentry, because you can effectively control the amount of structural load at max-Q speeds by wings (which have much larger L/D at max-Q speed compared to speed during maximal thermal load)
Yep, that was Len Cormier's (
https://en.wikipedia.org/wiki/Len_Cormier) concept with the "Windjammer" and "SpaceVan" light-weight concepts. But I'm going to note it wasn't as straightforward as it seemed and for the X-15B it was not really likely to be all that "light-wieght" even if they could have gotten it down to the suggested 9,900lbs/4,500kg M/T mass. The X-15A-2 was over 17Klbs/7.7kg all-up with ablative coating and all and the ONLY way to get the X-15B in the ballpark was to completely gut the hull and have no on-board propellant, which negated the 'advantage' of supposedly using the X-15 itself. And the natural L/D of the X-15 is still a problem especially higher up. Which is also why the X-15 had issues with high angles of attack in the first place. Pile that on top of the need to completely rebuild the X-15 airframe pretty much from the ground up...
Likely you correct. On place of pilot of hypothetical X-15 in orbital flight, i would not risk (unpowered) landing even if airframe is still perfectly controllable after reentry (which is unlikely due inevitable heavy damage - assuming here the airframe and pilot are still in one piece). Would just press ejection button as soon as speed is subsonic and let engineers work with the wreck.
Well, it's not like you had a choice since there wasn't any landing gear
Lifting reentry does not reduce total amount of heat, but allows to make heat pulse longer, therefore more waste heat is re-radiated from the back of wings and final wing temperature is lower. Also, early poly-phenolic ablatives had quite narrow range of effective ablation temperatures (about 2500C), and lifting reentry allows to keep the ablative close to optimal thermal regime for longer, therefore thinner coats are possible. Not sure if knowledge for optimal reentry profile planning was available in time though. Soviets get robust understanding of ablatives behavior only around 1966 (and used it in Soyuz series), after the series of extensive experiments and reentry tests. US may be getting the knowledge at around the same time.
The ASSET and PRIME tests, (
http://www.astronautix.com/a/asset.html,
http://www.astronautix.com/p/prime.html) tested some of the ablative formulas and worked OK. Though those were on lifting bodies not wing-body vehicles and that makes a big difference. The US did a lot of ablative testing in the 60s and for the most part they can be done but the hope was metallic or ceramic systems would be the better option. Actually the BEST option has always been metallic/ceramic, and active cooling system and a high-heat soak air-frame but as those are expensive, (and there's a real dislike institutionally for active cooling systems) so there is not much preference for them even for a 'reusable' system.
The "spray-on" ablative they tested on the X-15A-2 took over 700 hours to refurbish, was VERY tough to get off and not very effective over 'complex' surfaces like wing-body joints and protrusions. It also didn't wear well and left residue that had to be taken off with a grinder. Really we didn't see any really effective, "easy" to use ablative coatings till the mid-to-late 70s and the issue with use on something like the X-15 is still getting it to evenly ablate on lifting surfaces, (so the lift coefficient doesn't greatly change during reentry) and not getting "hot-spots" on the vehicle. The higher but shorter heat pulse actually is better for ablatives which is why they work for things like capsules and something like Spaceship One.
And none of the ablatives are particularity 'easy' to work with, especially any that can take reentry heating from orbital velocities.
Now on a semi-on-topic aside; The history of the concept, design and building of the X-15 vis-a-vis the X-20 is pretty instructive.
Consider just the issue with mission and parameter changes. As I noted the X-15 originally had more focus on speed in the atmosphere than near-space operations but that was quickly modified before construction began. Especially as only three (3) vehicle were going to be built the large amount of 'hand' crafting that went into them allowed them to be pretty handily modified. And repaired considering how many times they got 'pranged' over almost 200 flights.
The X-20 on the other hand had to be almost perfect the FIRST time from the very start. Not only were the mission parameters pretty unforgiving, (orbital operations are like that after all) but the most basic design considerations were that it was going to be the first of multiple operational vehicles so it was both prototype and primary operational vehicle wrapped up in one. (Those familiar with the Shuttle may find this sounding familiar and there's a reason for that) "Operational" in and of itself means more airframes than a test vehicle, maybe not many more, (see Shuttle) but arguably more and as well they have to NOT be 'hand-crafted' even on an industrial scale.
The problem was the X-20 WAS very much a difficult and complex vehicle even compared to the X-15 and it was shown in the difficulty and expense of the project to the point it was canceled. One can even argue that had it actually BEEN a test vehicle like the X-15 with a limited production run and a defined 'test' program in place it likely would NOT have been canceled!
But that also points out the main issue with a defined test program and a limited number of test vehicles. You don't deal with significant failure well and unless you can replace any loses you WILL eventually run out of airframes or mission goals. NAA kept pushing for more advanced X-15s and while initially there was money and resources to re-build there wasn't enough to build more airframes and eventually even that ran dry as the Apollo Lunar program grew and the 'need' for high speed research took a backseat to the need for more directed and direct applications.
I'd of really liked to see a couple more airframes built and the program extended to higher speeds and altitudes but as noted above it would have taken a great deal of effort and eventually lead to needing what amounts to a 'new" vehicle for the higher end applications. And here's where the X-20 failed since pretty much anything it could do could be done 'cheaper, easier, and faster' using a ballistic capsule. I've seen it often that many consider the X-20 the "optimum" way to get "five astronauts and a couple loafs of bread" to a space station and frankly that's arguably and demonstrably false. Even if we don't include the glaring fact that it never flew.
Mercury was never designed or built to be reusable but it could have been done within the basic design. The same was true of Gemini and Apollo. They never were because there was no need for it, because the main requirement was "soon" rather than anything for sustainability. It is "assumed" that the X-20, being a winged, reusable design would reduce the costs over the ballistic capsule and at first blush that looks true. But only because there was no reusable capsule design data at the time to be used in a counter argument. In fact a much less demanding airframe (X-15 post-program study above) when studied closely shows the majority of the assumptions are on shaky ground to say the least. Especially today when we actually DO have some reusable capsule data for comparison.
In an alternate time-line I'd wish to see the Air Force pull its head out, (trust me, as a career Air Force person "I" guarantee they very much tend to be the type to miss the forest for the trees as an institutional imperative

) and realize that no one on the political side is every going to give "space" straight up back to the military as they wished. (As it was they didn't figure this out until the early 80s when "surprise" they finally realized that YES having agreed to use the Shuttle for all spaceflight means those in charge are going to actually MAKE you use the Shuttle! Now who's laughing up their sleeves over those 'annoying' requirements you made NASA agree to meet for your support? Ha!) Hoping on an extended X-15 program that progressed UP to X-20-levels was likely a much smarter choice as it would have allowed incrimental progress that would also be well visible AS progress for the money and effort spent.
For example the idea of using the stored Navajo G-26 boosters, (none of the G-38 were ever completed but it would have not been difficult to re-build some later in the program) to boost the X-15 or X-15A2 into higher performance flights is very interesting. Granted the performance of the G-26 isn't that great, (
http://www.astronautix.com/n/navahog-26.html) it boosted the almost 65klb gross mass Navajo missile, (compared to the X-15A-2 gross mass of a bit over 35Klb) to Mach-3 and about 43Kft altitude. Call it putting the X-15A-2 to around Mach-4 and over 50,000ft before the vehicle itself adds somewhere between 1600 and 2000m/s velocity on its internal tanks. (The external tanks really added a lot of stress and drag to the vehicle which is why it couldn't hit the expected Mach-7/8 speeds)
No you don't hit orbit very soon, (or very easy

) but it makes a clear progression with very specific lessons learned and data built up. And in doing so you probably don't lose sight of the end goal which is arguably what happened with the Shuttle. (Don't tie all you "requirements" into a single vehicle since quite obviously if you lose it or any capability that comes directly off your overall capability. Again something that should have been a 'lesson-learned from OTL's X-20 since loosing the X-20 didn't materially effect the Titan and the capability could be replaced and/or expanded on demand. The same could not be said of the Shuttle both due to design and due to policy/politics)
Frankly anything above Mach-8 is going to require a full redesign and rebuild of the basic X-15 but that would be a 'logical' progression in this case and quite defensible in a TEST program but not so much in an operational system. This way by the mid-70s if you haven't reached orbit you'll have it in sight.
Randy