If your DH instructed him to check the entire roof & he charged you for that, I'd definitely expect him to return to fix it or refund your money. If your DH & he agreed on him trying to seal a targeted area, I doubt you'll get much more out of him. I can't imagine he's guaranteeing the entire roof, if he wasn't paid to check the entire surface & repair all leaks. He's probably only guaranteeing the section he worked on. Unfortunately, from what you've said, that probably isn't the problem area. It sounds like you've paid multiple people to fix the same non-problem area. That has to be extremely frustrating. I don't think I'd pay anyone else to attempt to seal that area. I hope you can find someone who knows what they're doing & will check the entire roof for leaks. Hopefully, you won't need to replace it. Good luck!
ETA: It may be worth your while to have your DH & DS('s) to spend a day looking for the leak. They could find tips on Google. My MIL paid three people to seal a leak on her roof, but it still leaked. After the 3rd person, my DH told her not to pay anyone else. He told her he would fix it, when we went up there to visit. He spent an entire day looking for the leak. He finally found it & fixed it. It wasn't in the same area the others were "fixing". DH isn't a roofer. He's not even in the construction field. He was simply the only person willing to take the time to thoroughly check the roof & find the leak.
FWIW, my MIL also has a metal roof. DH recommends checking the screws that hold the sections togethers & checking above the chimney. He say the screws can have rubber seals that dry rot. He mentioned other ideas, but they're over my head. If your DH would like more help trouble shooting ideas, I will have my DH read your posts. Our DH's could use our accounts to PM each other & try to solve the problem without costing you more money. At least, it's worth a try.