Unless there are some specific PA laws I don't know about, I recently went through this entire process for a friend.
One friend died and his sister was the executor of the estate. There may be a time limit involved, but it's long enough to do the whole process. The executor/heir can maintain legal possession throughout the entire transfer process. No dealer needs to be involved in an intrastate transfer unless the parties involved need a third party for storage or escrow or something.
The executor mails off the Form 4 paperwork with the buyer's info and registration fees. When it is returned with the stamp(s), the executor hands them over with the firearm(s).
If the heir is keeping it for themselves, it is a free transfer, but still must be done with paperwork during the time limit.
Here's the ATF letter that you probably need to start with: https://www.atf.gov/file/97596/download