For the Speaker of the House to ascend to the presidency, both the President and Vice President would have to be incapcitated simultaneously as in the event of an accident or attack.
If the vice president ascends to the office he has the right to appoint a new vice president without approval of Congress. Just as Nixon appointed Gerald Ford after Spiro Agnew plead nolo contendre to corruption charges and was forced to resign. After Nixon resigned the new President Ford then appointed Nelson Rockefller as vice President.
Thus if for whatever reason Pence becomes president due to Trumps removal, Pence gets to appoint a new vice president....leaving Pelosi and the dims gnashing their teeth and wailing away.
Pray for the President and the Vice-President.
Too much evil out there.