I doubt anything is going to come of the investigation. It's all investigation of what was done before he was president.
Nixon was impeached for what he did during the presidency. That's what Muller needs the questions answered. He needs perjury, obstruction, abuse of power. Maybe he could show that firing Comey was an abuse of power, I think some of the questions are geared towards that.
It's all a big nothing burger what happen before Trump was president.
Impeach him, yeah if you get enough Democrats up there. Again, whatever Mueller finds out will still be pretty inconsequential in that case as well.
Mueller's investigation is irrelevant to removing Trump. The President should just ignore it, go on about his business and wish them the best of luck.
Probably won't do that, and may very well do something stupid that gets himself impeached. Still Mueller's investigation is irrelevant unless he just happens to catch Trump in the act.