there are a lot of missing or glossed-over steps and choices to make.

I've used IntelliJ and Scala before, but regardless, I found these instructions difficult to follow and I felt very uncertain whether I was doing it right.

I'm skeptical that a new user could follow these instructions without a lot of guesswork and/or outside assistance

See the Nextcloud documentation concerning proxy configuration.

Expected behavior:
Emails containing notifications for new activity use overwritehost and similar values to construct the URL to the relevant poll whenever these values are defined.

Actual behavior:

A screenshot of the GUI, and some example programs, of which I have now a lot:

(Sorting by size to get a pseudo-random smattering, due to PNG compression, further down there are less complex images (in terms of data or entropy or whatever), esp