The problem with this scenario is that whatever Mueller finds as "high crimes and misdemeanors" will be so flimsy (firing Comey equals obstruction of justice?) that it will be hard to get enough Congressmen and Senators to commit political suicide to impeach and convict Trump.
Unfortunately when the swamp realizes that they may decide on a more lethal course of action to get rid of Trump.
You may be correct. Whether a sitting POTUS can be indicted is an open question.
I do not think the Reps in Congress will impeach, unless they think the evidence is crap and they want that fact known to the public by way of a Senate trial.
One more thought. Mueller may indict and force Trump to file a motion to dismiss, which will be denied by the Dem trial judge, who will be affirmed by the Dem majority in the DC Cir.
Then it will be up to the Supreme Court.
Were Trump to be indicted, I think it will be for a financial crime pre-dating his election, thus complicating the legal issue you raise.