Actually to accept a pardon would merely imply that you believed it was likely you might be found guilty, not that you accepted that you were actually guilty. It's different from. The situation where a pardon is offered to someone after they have been found guilty, when a condition of it being offered is tgat guilt is accepted.

I don't suppose it would work to claim that the Senate trial, if successful, was confirming the view of the House that Trump was guilty, and that the verdict should therefore apply from the moment of the impeachment, and that any pardons from that point on would not be valid. Since there has never yet been any impeachments that were confirmed by the Senate, there are no precedents to guide the court. It would be worth trying this anyway.   

I don't think there is any reason to think the supreme court justices slipped in by Trump would feel any obligation to help him. As originists they'd be likely to be trying to channel what the Founding Fathers intended. And it seems pretty obvious the FFs would have seen Trump as a contemptible little man.