The problem with your argument is that they weren’t indicted until after he left office. You cannot pardon someone who hasn’t been charged, indicted or convicted.
He absolutely can. It was widely discussed if he had the nerve to do it while he was still president since it would be seen as a tacit admission of criminality, but he passed the opportunity.
If by some extremely improbable turn of events and Biden loses next year - maybe if Manchin runs 3rd party, he will pardon himself for any future prosecutions on the way out the door. It will all be legal.
Sure you can. Gerry Ford did it. Blanket pardons are a real thing. Jimmy Carter did that.