white enamel yellowing

Long, long time ago Testors whites indeed used to yellow, but they fixed that problem twenty or thirty years ago.  Like stikpusher, I have some decades-old white models, and no sign of color change.

Same thing with clear overcoats.  Some of them used to yellow, but that was long ago.  Newer formulations are fine.  I would still not display models where they are exposed to pure direct sunlight.  Ordinary window glass stops most of the UV that causes color change. So indoor environment is fairly safe.

Not in my experience. I have some kits painted with white enamels that are some 20-30 years old at this point and show no signs of yellowing. Now decals, clear coats of flat and gloss, etc. can change that, but basic white enamel paints will last just fine on their own. The biggest culprit of any yellowing in my experience are Testors rattle can gloss and dullcoats.