Can leafpad, sct, etc. be installed on UT?
I'd love to have some of my favorite programs that I use on Debian, on UT. Is Libertine the only way around this?
Yes, try Libertine. Usage will be difficult unless you attached a mouse and keyboard via bluetooth though as these programs really aren't designed for small touchscreens.
Libertine is the official way of doing this however it isn't really in a good state right now even on normal android-based devices. It hasn't been maintained much especially xmir. Current state on the pinephone is worse but it could be actually better eventually once xwayland works.