It depends on how irresponsible Congress was being. If you start hearing calls from the likes of McCain, Graham, Sasse, and Rubio to impeach, and the House still hasn't done anything, McCain himself (or more likely Warner) might take the evidence to the NY AG himself.

I don't think it would ever come to that, but I don't think it's a fantasy either to think that the FBI case isn't gearing up to not only show impeachable offenses, but is gearing up to bring criminal charges.

Nixon was days away from impeachment when he resigned, and there was a criminal investigation that was leading to charges against him. He resigned more because of the charges and less because of the impeachment--impeachment and resignation have the same effect, but the resignation gave Ford enough cover for the pardon.

It's a very unlikely scenario, yes, because the House will likely move well before that so as to save the embarrassment of watching Trump get arrested. More likely, if the case was strong enough that they could give the material to the NY AG for an arrest warrant, House Republicans would rather go ahead with impeachment so that Pence could play the Ford role and "save" us from our "national nightmare".