No, @queen - you're wrong I think. For all formal events, its in the order of closeness to the throne (may well be different privately). So, for example, at a state dinner at which Charles&Camilla, Sophie&Edward, Kate&William were all present, precedence would be C&C, K&W, S&E.
If Harry & Meghan were there they'd come after K&W and before S&E.
Theoretically, I'd assume jewellery would be the same, but I honestly can't see this as a big deal. I imagine that on entry to the family and at your first big event, you pick one or two tiaras you like and if no one else is using them, those become your go tos. It's not like we see Camilla wearing one and then at the next event Sophie is wearing it. So, if anything, for tiaras anyway, it's more like first come first serve.... But the really good stuff (tiaras, fancy jewellery etc) only really comes out for the big events at which the core family are involved.