1. To convince him to let-up, outright, through these letters will be pretty dang hard. Im thinking like a 6 Obs? He has an absolute need for a scapegoat right now that would require some expert persuasion to discount, and there are also behind the scenes circumstances that would present a Disadvantage for you. Failure condition would be he isn't convinced, but would have a seed of doubt planted, and would also take them directly to Nicastro.
2. Can you propose something for Tomas to do that can Help with the test? He's not really a written prose kind of guy so I don't think he'd have an applicable Skill, and he doesn't know anything about the situation that you haven't told him, and so is kind of just a different set of handwriting to mask the ruse should it have to hold up to some technical scrutiny, right? As well as of course getting the letters directly to Heimart.