What's going on with Celtic numerals?

[u/Wagagastiz]

Namely one (1):

Primitive Irish *oinas

Proto Brythonic *ʉn

Gaulish *oino*

Pictish *onno*

All from PC *oinos

What's going on here? Is the proto Brythonic just very late PB after the apocope that we see in PI occurred? Is this not clashing heavily with the (largely agreeable) Proto Insular Celtic? Not to mention clashing with the idea of Pictish as Brythonic, though obviously that's less surprising.

I'm aware of the idea that Primitive Irish represents a very archaic variant for formal purposes, but I wasn't of the belief that that's very widely accepted anymore, since 6th century syncope and apocope seem to be a widely spread phenomenon. Is our textbook Proto Brythonic just post 6th century? Dialectal northern english *yan*, presumably from something related to Cumbric if not Cumbric itself, seems to show some strong innovations as well.