I am pretty sure that coach Flip is the one actually working with Fields on any mechanics issues. I know that I have read several times over the years that Flip is one of the better qb coaches and really works on footwork. There are probably also some tweaks with the throwing motion, but so much starts with the footwork and base that I think more of the focus goes there initially.
The impression I got from a couple of Nagy interviews is more than Dalton is just answering questions and likely giving advice on cadence/huddle stuff, reads, film study, etc. I mean, I suppose he could be giving some advice on mechanics too but I would be surprised to find that to be the case.