I can't help with the #21 but I've used #5 forever (first used it in '76, still use it on all my repaints for the first 3-4 months). FWIW it's my overall favorite of the Meg's "pure polishes".
#5 is nonabrasive, think of it as a "glaze". It leaves a *very* little bit of protection..won't quite wash off in a hard rain but won't last through a regular wash either (unless you top it).
It can be wiped off while still wet or allowed to dry. Try to keep it off black trim (wipe it off before it dries if you do get it on some). #5 is about the most foolproof product I've ever used, I can't even think of any tips/warnings (other than the trim bit, and it's not like it does any *awful* staining) because the stuff is just *so* user-friendly.
You'd apply the #21 (or other LSP, I use carnaubas on top of #5 when I use it on cured paint) *after* the #5. The #5 will provide a little extra gloss and fill some minor marring and then the LSP seals the #5 in so it doesn't just wash off.
Not dumb Qs at all, not that many people use #5; it's sorta an obscure, overlooked product but *I* really like it.