Does this open up legal avenues for Trump to seek immediate legal remedies - such as getting SCOTUS to rule the whole procedure is unconstitutional?
I would think there are going to be serious challenges to any attempt to convict him after he is out of office. My read is the charges are moot because he is no longer in office and are dismissed.
If you could do this, what is to stop a future Congress from retroactively impeaching Obama, Clinton, et al.? It is somewhat absurd.