Trump is not IN any legal trouble. Nor has he ever been, which is why the Dems have not been able to get or keep him out of offices
The problem here is that Trump may have done nothing wrong but sitting down with Mueller is likely to be used against him, probably to set up a "perjury trap."
Every competent defense attorney in D.C., and probably everywhere else, knows that--even if you have a completely innocent client--when the prosecutor wants to interview that client, especially late in the game (i.e., after taking statements from other witnesses), the prosecutor isn't interested in "clearing the air," but in trying to get the client to incriminate/perjure himself. Even if the client tells the whole truth, the prosecutor will charge the client with "lying" if any of the other witnesses can credibly contradict the client.
The defense bar won't take Trumps case because they can't get Trump to accept that is what Mueller is up to.