Just to get a few matters straight, if a DF'ed person lived far enough away from family and the family didn't know the elders (or anyone ideally) from that congregation near his home, this could be done.
Once the "old cong." JC trusted that the guys on the phone were ready to reinstate Bro. Fictional, a few phone calls later, they could sign off on the matter. "You guys met with Bro. Fictional, so we will announce his reinstatement and our secretary will send the updated form to Bethel." End of that part of the matter. Family hear that Bro. Fictional is a brother again, the secretary sends the form and drafts a letter of introduction to send along with any service records they have to the "new cong." It doesn't matter to the "old cong." what happens to those records now, it's finished for them. Mission accomplished.
As far as people checking on Bro. Fictional, some kindly C.O. or elder might do so. "Those contact numbers were changed, huh how strange." Or maybe the fake cong. elders keep the numbers and keep up the charade just in case. Or family wants to know from the cong. how Bro. Fictional is doing. Either way, Bro. Fictional says he moved again and hasn't been going to the hall. He does a lightning fast fade. It's done. He shouldn't tip off family that he's apostate, but he can speak his mind without saying WTS is a dangerous f*cking mind-control cult. I know I speak my mind about it to the wife and some to the mother, and they don't "turn me in." Same could be true for a DF'ed family member that got reinstated- family doesn't want to lose contact if they can find a loophole.
Having been an elder, I can say there isn't really any good follow-up on these things. Elders usually act aloof and don't want family to get involved in elder business, so they don't tell them phone numbers or anything, they just tell family to ask Bro. Fictional anything they want to know. I know I have called other halls looking for information and usually received it if I said I was an "elder" and usually was told that they would have to look into it if I said I was a "brother." That shows trust among the various elders. It also shows that family and concerned ones will get stonewalled if they ask elders for specific information.
Bethel won't want to close up the loophole by making itself more involved in the process. I could see them say to check on a congregation and call the P.O. according to the Bethel-approved contact phone number, but I highly doubt even that would happen. TRUST in the brothers. Plus, if a Bethel-approved contact number were published, the fake elders could call the number and find the one that rings and rings and rings with no answer and use that cong. What will trusting brothers do- call that number again and again or simply call the number those brothers gave them when they called? Just trust them.
Can it be done? Yes. Can it easily be foiled? Yes. Will it? Each case can come out different. Should it be done? Who are we to decide for someone else.