First of all, thanks for your replies! I'm certain that the tv hat is the best available solution given that it eliminates the USB interface just for starters but this isn't the scope of this thread. I have a XBOX One tuner and I'd like to get it working, sure that it is possible to get better results than the ones I'm currently getting, maybe not perfect but certainly not completely unusable like it is now.
I've made some tries dowgrading the client to version 188.8.131.52 (the oldest available in the libreelec repository) to no avail: the behaviour remains EXACTLY the same. Then I've dowgraded the server to version 4.3-1804 (again, the oldest available, this one not in the repository where I couldn't find any but on the nightlies server) and things have even worsened. Now it gives IMMEDIATELY the error message that the server isn't responding properly in spite of the fact that before downgrading I've uninstalled, cleaned all and reinstalled the older version from scratch.
I'd like to try a 4.2.x version for the server but I don't know where to find it given that in the official repository I find only the client, not the server. I'm starting to think that the problem aren't the server and client versions but libreelec 9.2 as others report. Downgrading to 9.0.2 would mean reinstalling everything and this is a pain but I would do it as soon as I would have some spare time.
For what concerns channels numbering, you're right and I know that but I had expected a right numbering from the outset just as it happens with every television because doing it manually is very time consuming: I'm certain there is a smarter solution which I miss because I tend to exclude a great piece of work like tvheadend could have such a big limitation. Have a nice day!