I feel like this sounds worse than it is because you also don't create a Threads account, per se, in the first place. You use your Instagram account. If you want to use Threads, you activate your Threads profile; if you want to stop using Threads, you deactivate your Threads profile. Your Instagram account is still your Instagram account and exists independent of Threads. If you want a totally separate account for Threads, you need to create a second Instagram account for it.
Nah, it's pretty clear when you open Threads and it says literally "Log in with Instagram". It's like logging into YouTube with your Google account and then being mad that deleting your "YouTube account" deletes your Google account. You can always disable your YouTube or Threads profile.
This is messed up. You'd think it would be as easy as turning on or off. It should be that way! This guarantees that regardless of functionality and/or popularity, if users tire of the platform, Zucks still maintains their account unless you decide to ditch both.