Yes, documentation is certainly an area where TurnKey could do much better! Over the last few years I have been making efforts to improve our docs. Although there are still so many gaps that a casual TurnKey user could be forgiven for not noticing that!

As you note, we use Transmission as the torrent client and use the default WebUI that it's bundled with. In theory, you should be able to apply the upstream (i.e. developer's) docs to TurnKey. Having said that, we install the software from the Debian repositories.

The major benefit of that is that security fixes are backported by the Debian security team and are auto installed by TurnKey servers. This means that the chances of changes breaking things are extremely unlikely. On the downside, it often means that the version we're using is lagging behind what the upstream developers provide. So sometimes that means that upstream docs are ahead of what the local version is which can sometimes create confusion.

Regardless, I just had a quick google and my reading suggests that the only way to support multiple users, is to run multiple Transmission daemons (one per separate user required). Unfortunately I didn't find any clear tutorial on how to do that that seemed like it would be 100% relevant. Many are for Ubuntu, and whilst Ubuntu is also based on Debian, it's not binary compatible, so there may be issues following an Ubuntu tutorial (for example, never add an Ubuntu PPA designed for Ubuntu, unless someone explictly says that it supports Debian too). But (at least in theory), with a bit of basic Linux/Debian knowledge it shouldn't be too hard. Although without actually attempting it myself, I can't really guide you on how to achieve that.

In essence I imagine that the procedure would go something like this: create a new Linux user, then replicate the current service file (I don't recall whether it uses an init.d script of a systemd unit file) - obviously it'd need to be named something different to default. You'd then need to adjust the service to use the new Linux user account, as well as the port the additional daemon will use (you can't have multiple daemons/services listening on a single port). And there may be additional requirements too, I'm not 100% sure.

If you keep in mind that TurnKey is based on Debian (v14.x was based on Debian 8/Jessie - our upcoming v15.0 appliance will be based on Debian 9/Stretch) then hopefully google will help you out with more of the details.

Sorry that I couldn't be more specific about your specific aims, but please feel free to post back with further questions.

Actually, FWIW this link looks like it may be ok?! I haven't read over it extensively and I wouldn't install from the PPA as recommended (unless you had confirmation from somewhere else, preferably from the PPA maintainer) that it's ok on Debian. But from a glance, other than that, it looks like it may be ok?!

If you want to give it a go, I suggest that you create a test VM server. That way, you can document your progress and if things go really pear shaped, you can trash your server and start again.