As a rule of thumb, one should try to stay at the job for at least six months from the date they are granted permanent resident status.

Yes, but it's only good for four months. The one based on a pending adjustment is valid for two years.

If it's a multiple entry, they can return before it expires. If not, they will need another way to come to the U.S.

When you get the decision, review it with your lawyer and decide what to do.

She can send a letter with a copy of her ticket showing she left the U.S.

Adjustment based on marriage to a U.S. citizen does not require one to prove that they are in lawful status.

The short answer to your question is that you do not file the I-864 with the I-130.

It doesn't look like you have any bar to admission, so I don't see why this would prevent consular processing or any issue using the drop box.

You probably did not have to file two applications, but there is no rule against doing that.

It is not a problem to your application for extension of status, to renew your passport.