OK, so when you lower the temperature until you get an unsatisfying vape there is still burnt flavor but less of it, and you think the coil is reasonably clean.
IMO that head is toast unless you clean it out and re-wick it if that's possible to do with these heads. It's either not as clean as you think it is or you've burnt the wick in it and the wick is giving you that off flavor now.
With your nickel coiled head, 'no atomizer found' probably indicates the resistance is too low. What does it read on your ohms tester? Andria might be able to help more with this too since she has one of these mods.
Either way, you need to buy more heads because they only last so long until they get gunked. They can also short though that doesn't happen often, but I don't know these heads.
Well, I don't know anything about nickel, or about "replacement coils," since I build my own. No amount of awfulness from commercially-made coils surprises me though, and that's why I started building my own -- I refuse to pay good money for a coil that's had zero QC applied to its manufacture -- I am my own QC. You know what "they" say -- if you want something done right...
Andria