It's far from clear-cut that applying for or accepting a pardon constitutes an admission of guilt. That idea comes from a line of dicta from
an old case where it was originally meant in a different context, and a federal court fairly recently
rejected that interpretation. And even it were firmly established that accepting a plea bargain constituted an admission of guilt, why would Congress impreach him for that? They don't have to impeach him if they don't want to, and I think Republicans have made it very clear by now that they will stand behind Trump no matter what he says or does.
But of course, he doesn't really need a pardon. Once he's the president, he can make all of his federal charges go away, and even if he's immediately followed by a president who's willing to resume the prosecutions, it'll be years before they can bring charges against him again. Honestly, even if Trump loses the election, I think it's very likely he'll be able to continue to postpone his trials until either he dies or the government figures he's too old to be worth prosecuting. The Supreme Court have shown that they're willing to play along with Trump's ridiculous delay tactics.