1) I think that's by design: you've got two ways to use OMEMO, the "friendly (default)-way" (with BTBV, Blind Trust Before Verification), and the "paranoid-way" (where you verify each and every device). In the latter, you don't want your messages to be encrypted for a new, unverified device (as it could potentially be that of an attacker): you can either verify the new device yourself, or your contact can verify theirs and you verify them again. Like I said, that's not for everyone. And if you are not that paranoid, then verification is an afterthought anyway and you don't lose or gain much with verification anyway.
2) That's where MLS¹ kicks in, essentially a new encryption scheme that's suitable for large group chats. There are projects² for its adoption in XMPP. I have no strong opinion about MLS, but I tend to think that it's more for the gimmick and having XMPP score well in random protocol benchmarks: if you've got such a large groupchat that OMEMO doesn't scale anymore, you have a groupchat with so many participants that it is only private in name and more or less encryption doesn't matter.