I would flip 3) and 4) in terms of plausibility. The problem is that the S-IC/S-ID is very expensive and out of production, so if they're not planning on making it the centerpiece of their program they are extremely unlikely to choose to use it as the booster for a Shuttle orbiter. According to the information they have at the time, solid rocket boosters are cheaper and just as good, therefore better than Saturn-Shuttle. It's only with hindsight that we long for keeping the S-IC around.
I've been doing digging into the costs of a Saturn IC (and the other Apollo era hardware). I am having trouble finding primary sources that give costings on the individual elements of a Saturn V launch. (Such as the costs of the launch itself, the costs of each stage etc.)
That disclaimer in mind, from the secondary sources I have found, it does seem that there was scope to really drive down costs on the S-IC and S-ID. For example, the F1A engine looks like it would cost around half what an F1 engine cost (I have seen secondary sources give the cost of the F1 engine as $6.9 million in 1969 dollars, while the F1A looks like it would have cost around $3.9 million in the same year). And since the F1A engine had much better thrust than the F1 engine, one could turn the S-IC/S-ID into a 3 or 4 engine stage (depending on what payloads and upper stages you were putting on top of it), which would drive costs down even further.
So the S-IC/S-ID could be worth returning to production, depending on the situation.
On the other side of the scales, the flyback S-IC would have rather large technical risks. Boeing's proposals seem to involve more than a bit of handwaving, so while I suspect it would have been possible to make a flyback version of the booster, I have a feeling that development costs would have risen above Boeing's promises.
(
@Archibald,
@RanulfC,
@Shevek23, @anyone who obsessively collects technical pdf files: Speaking of digging into the costs of the S-IC, does anyone know where to find the 1992 Rocketdyne study on putting the F-1A back into production, it is refereed to
here and I've been hunting for it ever since.)
If they are going to keep the S1C at all they only way to "sell" it is to go the RSU route. Cost is an issue of course and 'selling' it to OMB is going to be fun
Was the OMB really so important? Sure they wore NASA down, but in the end they lost and Nixon and Weinburger approved an impractically large shuttle that required impractically high flight rates to pay off its costs.
1) What 'surplus' stocks?

Seriously you'd have to put them back into production to 'use' as we only had a few.
I thought there were a handful of unused S-ICs around? With 5 engines apiece, those would last NASA a few years, as it's hard to see them launching more than 2 uprated Saturn IBs a year.
Unless you can get NASA to reconsider the Saturn-1 but they were focused on keeping something of the Saturn-V so I suppose they would have pushed for an INT design until the law was laid down....
I found
this interim earth orbit program study to be a fascinating insight into NASA's thinking about their space station program. The document is full of helpful costings of how much different boosters would cost and how much it would cost to restart different production lines.
According to this study, the "vehicle acquisition cost" (which I am assuming does not include the cost of launching said vehicle) of a Saturn 1B was $50 million in 1971 money.
By contrast, if NASA had developed the Titan IIIM, each LV would have cost NASA $25 million.
Now, the Saturn 1B was more capable than the Titan IIIM, but even it was a tad undersized for the space station missions NASA was looking at (where a LV that could get 50,000 lbs to a 250 nautical mile orbit was desirable, since such a LV could carry both crew and station resupply together as a combined payload) and it would have struggled to launch all but the most minimalist of glider-type shuttles. And while there were options to uprate the Saturn 1B, they seem to have been dismissed as too expensive (though that's not looked at at all in the IEOP study).
It's worth noting that the Titan IIIM would have been rather expensive to develop - $250 million in 1971, which is a good deal more than what it would cost to restart Saturn production and develop any of the Saturn INT vehicles. And certainly more expensive to re-start Saturn 1B production.
I think that is the real reason why NASA was so interested in the INT-20 variants (the S-IC with fewer engines and only a S-IVB on top) while the INT-21 was of interest because it was the most economical option for getting space stations to orbit. Downsizing the Saturn V hardware was cheaper than uprating the Saturn 1B to do interesting things
And I suspect that the attractions of the S-IVB combined with a solid rocket cluster as a first stage was a big reason why the Saturn INT vehicles didn't go further.
Based on the IEOP study and these Bellcomm memos (
19701127 S-IVB + UA-1207s--Bellcomm.pdf and
19690318 Low-Cost S-IVB--Bellcomm.pdf), it looks like the cluster of UA1205s or UA1207s would have been put medium sized payloads in orbit for much less than anything based on the S-IB or S-IC.
As I noted above even if NASA had wanted to develop the LRBs the political "will" wasn't there and the nature of most of the 'other' upgrades had similar issues. (The Aft-Cargo Carrier, for example wasn't needed because there was no known need for that additional cargo, and so on) And if one is going to make major changes to the "Space Transportation SYSTEM" so the idea was then you might as well go to the 'next' step as a launch system. (Hence NLS, ALS, etc, in fact it was my understanding that NASA didn't push the Shuttle-C because it WAS based on the Shuttle as a system in addition to being a direct 'competitor' to the Shuttle... How? It was an UNMANNED Shuttle

)
True. Though to be fair to NASA and Congress, the Shuttle as a launch system was very much oversized for America's needs. It was designed to be economical in to meet a demand of 60+ launches a year and for most of its time the US was launching maybe 20 things a year... Investing further in an oversized system is hard to sell.
Maybe the only way I can imagine the LRBs seeing the light of day is if NLS or ALS had gotten some traction and the first stage of NLS/ALS had been something that could have been used to boost the Shuttle.
Also, given that the Shuttle was so oversized for America's needs, I think NASA was right that any other big cargo LV would be a threat to the Shuttle.
fasquardon