Based on everything ive researched it doesn't look like the excessive donations is the serious part of his plea. Rosie O'Donnel had done this recently as well as thousands of others. Typically nothing is done about it. At most money is paid back to the donor or in the worst cases there is a fine to pay. Nothing indictable based on what ive been reading. This was a big issue for Bernie Sanders too but nothing came of it.
By "excessive donations", are you referring to the hush money Cohen paid to the women?
Well in this case we have a criminal conviction for just that, while his co-conspirator, and the person who instructed him to do it, was named under oath.
I believe the Sander's case you're referring to was in regards to some Australian's who offered to volunteer for his campaign? The difference there was that it seemingly was an honest mistake, and it involved something of relatively low value in comparison, so they simply paid a fine for it. Not to mention that Sanders himself probably wasn't aware of every volunteer working for him. In this case we not only have Trump and Cohen on tape discussing the matter (and there's supposedly more info to come), but according to Cohen's lawyer just now, Trump's lawyers wrote to the special council and told them that Trump "directed Cohen to make that payment."
So there seems to be no question or dispute of Trump's involvement. But his legal team would probably try to make the case that it didn't break campaign finance laws, if he were to stand trial.
And since Cohen was convicted on this specific charge, it certainly raises the possibility that the named co-constrictor of this criminal charge stands trial as well, if possible.
As I said in an earlier post. Clinton was impeached for lying about having sex in his office. In comparison, this seems like a bigger deal.