A jumper is a bad example, because it’s not just made up of sleeves.

A bra is a much better example. But I was never making an argument that all things made up of two parts are referred to as a pair, just my thoughts on the rationale behind cases where we do.

I’d say it’s less of a grammar rule and more about how things made up of two similar parts can be conceptualised

They’re made up of two similar parts which form a pair, unlike a jumper

Don’t see why there should be any political opposition from the West for Ukraine going deep into Russia. Russia is the agressor invading them, after all. Anything between Kursk and Vladivostok is fair game.