Congress seems to be quite toothless lately.

True but they will have a hard time ignoring the 22nd amendment of the US constitution and it would be an easy move to remove him from the election process which a majority of the states themselves could do without opposition.

They might try the good old Putin trick, have a puppet elected on behalf of Trump and let Trump have another high-ranking position (e.g. Vice President) and hold the real power. Trump can still do most of campaigning, and there is also ample opportunity for election fraud.