What can POTUS do to stop the steal? Well, besides sending in the lawyers, he could:

Send in the Federal marshalls to secure the polling places!!

Let the DEMS be the ones to sue. They can sue sue sue, but they won't be able to manufacture ballots in the open, the time will run out, and that's that. No fake ballots, no theft. Even if the courts say that the President can't do this, by the time they do it's all over.