Reneging a second time - risky for background checks?

I think you should stay with the FT offer with B, though others may disagree. My opinion is the most conservative view point, which would be the safest route but would leave some money on the table.

I think if you were still looking around and hadn't signed/accepted the full time offer, then this is quite straightforward, and you accept with C at the higher offer. The difficulty here is that you've signed an agreement which is binding in principle. While you could theoretically renege, having this come up as a background check could ultimately lead to company C rescinding your offer, leaving you without an offer at either firm.

It's quite a difficult decision, but I think it's an ethically sound one that should allow you to get to nearly the same position a year from now if your value is already at a 33% premium from a comparable firm. There's very little value in severing multiple bridges and contacts at various firms, because you never know when that contact could be useful down the line, or if there's someone at company C who knows the same at B.

Perhaps as an alternative you can reach out to C and explain your situation? It could help erase doubts, but it wouldn't necessarily be prudent if you were interviewing around after signing a FT offer in my view. Even if you have to let them that you've accepted at another, expressing your thanks for the opportunity and a receptiveness for future opportunities at the firm will go a long way. I'd love to hear everyone else's thoughts.