My colleague *may* have forged data. Any advice?
Talk to your PI and/or colleague. It might be wise to start with something less confrontational than the "I think [Colleague] may have forged data", however. Maybe "hey, I still want to understand what was going on with that figure. Perhaps other figures might be affected too? What do you think?".

Best case scenario, there is a convincing answer that can calm your nerves. There are a whole slew of other worse possible scenarios that can follow from this discussion, but they're all better than finding out after publication that your suspicions were correct. As for the revision deadline, it can likely be extended if need be. It'd be up to the editor, but I imagine few prestigious journals would say no to a reasonable extension request to double check something important.