Hey, GF, I've got a helmet here that I'm going to be doing some repair work on. It hasn't turned brown and I think I originally finished it maybe about 6 years ago or more.
It's not that I doubt Clint at all - he's a pro after all - but I'm sure there are a lot of factors that play into it. If anything, I think some aging would improve the look of the helmets. The screen-used helmet that I had for a few years was not a bright gold. It may have been much brighter in 1991 but some aging isn't going to hurt a replica (or an original) in my opinion unless it starts to really become unstable and you've got to intervene in order to save the prop. And as long as you keep the prop out of direct sunlight, it should be fairly stable in my opinion.
Clear coating will certainly dull it but if you saw the original helmet in person, you wouldn't worry much about dulling the gold paint down a little. It was a MUCH duller finish in person though I don't know if it was clear coated back in 1991 or not. I'm guessing it wasn't but it's just a guess.
As for clear coating turning it brown... I'm not sure exactly what Clint has in mind. Most gold or metallic paints recommend against clear coating because it dulls the finish. Of course, that also assumes you don't want the finish dulled.
But clear coats in general I don't *think* have THAT much of a problem with darkening or changing the colors of what they've been applied to from what I've seen in my years finishing Rocketeer helmets or with other various model building, hobby and art projects I've worked on. I've built plenty of model kits and I've never noticed any of those turning brown or any negative affects on paints underneath the clear coats. Maybe Testor's is good stuff? It's what I've always used anyway and I've never noticed any problems... Maybe somebody else on the forum might have some input on this?
If you're concerned about it, you could skip it - I don't think it's absolutely essential though I do like the way it makes the weathering and the gold appear more uniform.
As for the tape residue, I use masking tape to hold eye lenses in place while I glue them in and have left the tape on for a week or more at a time. I've never once had it leave ANY residue on the lenses and all I'm using is just your typical painters' masking tape. I think I've finished maybe somewhere around 8 or 10 Rocketeer helmets and I've never had a problem with masking tape. I've had trouble removing glue... and paint... and a few other things but never tape. Just avoid getting the tape on your finished paint job. It might mess that up.