I don’t typeset music myself but I’ve heard that LilyPond is very good. It is based on LaTeX.

I use Liberapay and have been pretty happy with it. Have found quite a few of the people and projects to which I would like to donate, but not all.

I use Runbox, and have for many years. Overall I’ve been happy with them. I think they have the features you are looking for.

I suspect that the Belgium-Netherlands border defies any mathematical description.

This seems quite reasonable to me, and if you are doing the more risky activities, then presumably you’d get some insurance to take the edge off the rescue charges.