n00b questions - can I add additional repos? how is UT different from a "full" ubuntu

[pizzalovingnerd]

1. Can I add additional repos to gain access to additional packages built for the arm64 architecture, or will it break my install?

Ubuntu Touch does ship with apt, and you should be able to add apt repos, but all of Ubuntu Touch's apps come from .click packages, which are kind of similar to APKs. Source

If you want to run desktop apps, or LibHandy apps or something like that, you could run that using Libertine

2. How is Ubuntu Touch different from a full GNU/Linux OS? I get the amd64/arm64 ISA difference, but what am I missing? 

The biggest difference is that it mostly uses click packages instead of apt, snaps, or flatpaks, and it ships Lomiri. Other than that, behind the hood is it just Linux.

3. Once the phone goes to sleep/screen shuts off, it loses wifi connectivity, and I have to restart the phone to reconnect to wifi...is there a fix for that?

I don't know the answer to this. Sorry.

4. Is it possible to install Phosh on top of UT, instead of the Unity/Lomiri shell?

It might be possible using Libertine, but I'm not sure. If you want a Debian phosh experience, I'd recommend Mobian.