The GOP had no mechanism to overturn the will of their crazed primary voting base. They thought they could always control the insanity they induced with decades of corrosive right wing agitprop. Crazy doesn't work that way forever.
They did actually and were openly considering it. There were lots of reports about them looking at how they'd just plain not put Turmp as their nomination. They of course knew that would splinter the votes as the Turmp fans would vote for him (and turn against the GOP, which, they already had in actuality; and the GOP has the gall to act surprised after all the shenanigans they've pulled...), and whomever they put forth had already shown to be less popular than him so they had zero chance of winning the Presidency by not backing Turmp, and I think they also feared it might cost them in Congress as well.
I don't think knowing he was being investigated would've changed anything about how the GOP acted, as I'm 100% certain they already knew he was. Obama had discussions with high ranking GOP reps (Mitch being one) specifically about the fact that the intelligence agencies were saying that Russia was meddling and that it definitely involved Turmp. Obama didn't want to come out with that without support from Republicans because he knew they'd just spin it as him having orchestrated it all (and it would've driven the frothing conspiracy nuts into overdrive).
Now maybe Romney and some of the ones that have at least some dignity would have seized on that to say no Turmp, but considering how quickly he dolled up to go out to dinner with Turmp, I don't think it would've changed anything as far as that goes.
I do think knowing Turmp was also under FBI investigation would've been enough to change the election itself though (I don't think it would've drastically changed the numbers, but I do think it would've been enough to push how the swing states went).
Although Obama's reaction to Crimea wasn't exactly strong, Trump has been trying to undo all of it, including crying at the G7 conference about the lack of Russia. Trump also continues to deny that Russia did any with the elections, Obama was too concerned with appearance but Trump is outright lying about it. The two aren't comparable at all.
Sorry, I have to disagree with your first assertion, I'd say it was plenty strong. In fact, it was strong enough that it was likely what triggered Putin pulling out all the stops (short of assassination of American politicians) to get Turmp in, in order to try and ease the sanctions.