To summarize==
1) no explosion at all, the slag holds=> another blowup somewhat later, but not too much later.
2) O-ring fails but on the far side from the tank--bad boost, thrust imbalanced, no way to abort anything=> possible mission continuation depending on how badly off the boost phase is, possible once-around abort, possible abort to African landing field, possible crash in the Atlantic which the Shuttle could not do survivably, so everyone would have to improvise jumping out of the gliding orbiter on parachutes, which I presume were handy, and with life rafts which I hope were handy. If the aerodynamics of the Orbiter in glide mode even allowed for crew to bail out. They'd have to do it one by one and be scattered miles apart on the open ocean.
No matter what, with or without loss of crew, the mission will be badly impacted and attention focused on the problem.
With loss of life avoided, the atmosphere will be less somber, but it will be necessary for NASA to adopt the new version of booster, meaning a hiatus in launches until the new design is finalized, ordered, and produced; it probably requires a campaign of firing tests too.
It could be worse than OTL, I guess, the O-Ring could fail within seconds of booster ignition causing the tank explosion while the Shuttle is still on the pad, wrecking the pad. In this circumstance, the blast from the tank explosion would impact the Orbiter worse, due to happening at low altitude, in denser air with lower, very subsonic airspeed. I don't know if it is known absolutely whether the crew was killed instantly when the blast wave from the tank hit them, or if they died later, of injuries during descent or with the final impact on the sea.
If they were known to survive for a while after the explosion I'd say they might have a chance with a pad explosion, but the blast being relatively more powerful, probably not. If they were known to be dead with the blast OTL, surely this surface blast will kill them too. If they could survive the initial blast, presumably the Orbiter would fly off like a kicked football, very likely breaking up, and if not, it will land going crunch somewhere, on land I think most likely because I think the Orbiter faced inward from the shore. That crash landing seems likely to finish the job of killing any crew left alive by the blast and spin.
No telling what the Orbiter would crash onto. What if it hits the VAB?
Meanwhile the tank going boom on the ground is likely to kill some more people I'd think--unless observer zones were set up with a most pessimistic estimate of possible blast energies, which they ought to have been.
So best case scenario, on this launch or an earlier one, an O-ring failure that produces a spectacular gusher of fire squirting sideways away from the fuel tank causes a degraded mission as the Orbiter reaches orbit but falls short of mission target. NASA cannot shrug it off, especially when people realize what would have happened if the torch had been pointed inward and that there is no particular reason it shouldn't have been. All launches go on hold while the new booster design is ordered and procured.
But with Challenger surviving, there is never any authorization to construct Endeavor. As the fleet nears EOL, there is no young, ultralight version in the inventory.