He was elected by the "will of the people", and there isn't any "no felons" clause anywhere in the Constitution. He also hasn't been impeached, convicted, and removed for high crimes and misdemeanors. Any pardons he issues will likely be entirely legal.

(We really should require pardons to pass a bipartisan Senate committee where there is no odd-numbered tiebreaker vote and require one defection across the aisle to approve--or something like that)

The pardon is from the office not the person is it not? Or that would open the door to question all decisions made during presidency, no?

Good luck arguing that with this Supreme Court. A future decision to the contrary wouldn't be applied retroactively anyway.

No one has seriously raised the possibility that Trump cannot issue pardons. There is no legal basis for the idea that he cannot.

Toward the end of his first term the question was raised whether he could pardon himself.