I assume you mean show her his command seals as a sign of our trust in her; only they are on his upper arm and not on his hand. Also showing her our hand sounds like a marriage proposal, when read out of context and truthful showing Rin that you trust her enough to show a 'rival' family member his CS, would likely be interpeted as another act of courtship with her as its would be a very deep sign of trust. One at the level that close lovers have.
Overall, this kind of write in, is basically what I would like to happen; a frank discussion that covers a gambit of topics. But I afraid that, if we voted for something that is long, complex and with multiple parts, we will get a huge backfire because we did not focus on one thing to talk about. Write ins usually suppose to be of the same complexity as the other non-write in answers. They're not suppose to be an include everything and the kitchen sink choice.
Combat training is always good but what kind of training can Rin give Shinji really? Without circuits he can not use anything she knows other than her martial arts training (if she even has that at this point). Plus the skills she is learning are from Touko and/or Tomas, sources that Shinji has already had a chance to learn from. All Shinji would get with Rin is another sparing partner (which I admit is a good idea).