> Can iPadOS display a UI tailored to the native resolution of the external monitor such that the user need never interact with the iPad's own display?

Well, since the iPad display is also the touchpad, you probably don't want to never interact with the iPad display. But essentially yes. Some TV's have a worse time than others. iPad's can't control what the TV can handle. In general, I've never had big problems, though I don't use it for 8hr work sessions.

> Is using a mouse with Mobile Safari a pleasant experience if the user is doing many hours of interaction that way?

If you are on macOS you can just scroll your mouse cursor over to the iPad and find out yourself. See: https://support.apple.com/en-us/102459

Nobody can tell you if what they have implemented now, works well enough for you. I use it regularly, it works great.

> (Actually, now that I think about it, iPadOS is too restrictive for me: I can't configure it in ways I would want to, but GrapheneOS doesn't have that problem what with being almost entirely open-source.)

backing out already?! :) Seriously though, you are not alone. iPadOS is restrictive, that is either a bonus or a curse. It does let you focus more on tasks, but it limits how you are used to working in ways that might be hard to handle(especially at first).

I agree about GrapheneOS.

As for emacs, you can run it under iSH on iPadOS. I can't tell you how well it works, since I don't use emacs.