If Hawks didn't want his name on THE THING FROM ANOTHER WORLD, why did he risk taking credit as a producer for it?
Kenneth Tobey, star of The Thing, told Fangoria in 1993 that Hawks gave The Thing to Christian Nyby to teach Nyby how to direct. Tobey said that the one scene Hawks let Nyby direct didn't turn out so well (he called it the worst scene in the movie), so Hawks taught by showing him what to do rather then letting him screw it up more.
Tobey said that Hawks directed the whole movie except for that one scene, so if Welles directed some of it he either did it on the sly while the leading man was hidden away somewhere else, or else Tobey was telling fibs.
What I've seen here is a lot of conjecture, but Tobey is the only person who was definitely involved with the movie (he's right there on screen) who I've seen speaking out about who really made the bloody thing. Which, if it was a cheap B-movie, was probably one of the two or three best cheap B-movies ever made, Welles or no Welles.
Flint: by your account of The Nightmare Before Christmas's parentage, wouldn't that mean that if Welles wrote a script and designed some sets and then someone else made it without him it'd be a Welles movie, but if a producer wrote the script and designed the sets but Welles personally supervised every element of the movie through shooting and postproduction then it'd be the producer's movie?
The settings and characters might well have made Nightmare memorable, but so did the incredible animation, the tone and the sense of humour, which were pure Selick. His attention to detail is astounding, and animation is all about the details. Nightmare had a shooting schedule of about a year - remember that it was made frame by frame entirely by hand - and by all accounts Burton spent a total of about five days on the set, mostly doing publicity.
I remember a Rick Veitch comic where an artist blows up comic strip panels with a projector and traces them onto canvas, and actually hires the original cartoonist to do the tracing for him, then claims that he himself is the real artist for having the idea but the cartoonist is just the monkey who executes it.
Poltergeist on the other hand is a different story, as it was by all accounts directed by Spielberg at least as much as it was by Hooper. Spielberg had to take out an ad in (I think) Variety apologising for claiming to have directed it, but cast & crew members have said that those claims were true, and several people have hypothesised that the only reason Hooper was involved at all was because Spielberg was worried about having too many movies out at once. Never mind the Poltergeist and ET were never going to compete for an audience.
What I'm trying to say I guess is that the answer to your "Is it a Welles movie?" is probably even more complicated than Glenn's four-tier system, due as much to the weird collaborative nature of film as anything else.
People say that The Third Man isn't a Welles movie because he didn't contribute to the directing - even he said that he didn't - but imagine if Noel Coward had played Harry Lime, as (I think) Selznick wanted; the whole movie would be completely different. But in Bert I. Gordon's Necromancy, if you replaced Welles with any other actor the movie would still be just as shitty.
As far as I'm concerned clever camera angles and technical skil that Welles may have brought to other director's movies when he took over/helped out/whatever are all very well, but unless it's constructed in a Wellesian way and is about Wellesesque ideas, it just ain't Welles. You can draw a clear connecting line through everything from Kane to Fake, and movies like Black Magic and even The Third Man won't appear anywhere on that line, though movies as vastly different as The Trial and Chimes At Midnight will.
This has been an undisciplined ramble, sorry.