So, if we go for the "Aer is borrowing/stealing mana from the 5 monocolour planes" storyline (which I think is a strong probability at the moment), I think the Crests designed here: Heavy Mono Signets & Idyllic Crest Sea Crest Gorgon's Crest Arctic Crest Fungal Crest should be a strong candidate for the objects used to do it. I envisage the Aerans dumping these over the planes and tapping into them when they need to (before the conflicts)

I would be alright with that, though originally the Aerans were unaware of the presence of the others.

If the mana stones are common it would suggest the aerans scattered multiple stones of each type across the relevant areas, but that works as well in the story as having five large ones. I like it, it makes sense to tie together the things we've already thought of.

(There could be five large ones as well, if we want rare colour-aligned artifacts later, either mana fixing or something more splashy).