For instance Picard appeared to come over a big part of his trauma by the end of "I, Borg" by dealing with Hugh, which brings up the fact that the Borg shouldn't be a collective anymore because of his return to them. Yet there is, without any in-movie explanation.
This shows a gross misunderstanding on your part of how people deal with trauma. Trauma never really goes away you just learn to live with it. But there can be times that certain events can bring all those compartmentalised feelings rushing back. Picard being faced with a Borg invasion of his ship and seeing his crew assimilated first hand would absolutely have caused him to react the way that he did.