This is really becoming compicated, huh?
I think it's often a good idea to look into the things that happened just before the car began having trouble. You seem to think that the fuel additive may have influenced this issue, and while that usually should not have caused this type of a problem, it seems an odd coincidence. I wonder if you don't have injectors that are stuck partially open for some reason. Are you getting any type of check engine light(s) coming on? Bad airflow meter would typically cause them to come on. It is just so unlikely that you have gotten 3 defective sensors. And the head has been off, so mechanically, we should be able to rule out the engine (as it relates to combustion). Perhaps you do have a headgsket issue, causing a vacuum leak, which could contribute to all sorts of rich running conditions and fouled plugs. The intake must have been off during head removal, is that all sealed properly? Pour or spray a little water over all areas that have seals/gaskets between them while the engine is cold and running, look for bubbles.
I hope this might help a little, hang in there!