"but also It leads me to think that this information is not essential to discover the instance that starts TVh as root"
This is hard enough even with the extra info. The extra info doesn't mean the problem is solved, but it sets a much clearer context and, as Jonas explained, allowed others to have a certain window into your level of expertise, which also helps.
In hindsight, and I realize not possible to know everything in advance, would likely have been better to try to "cleanse" your broken system of tvheadned after uninstalling the tvheadend packages with apt / dpkg. Using the variants on find / locate / lsof commands I mentioned.
"I uninstalled with apt-get remove --purge and with dpkg -r and finally both reported that TVH was not installed"
Yes, but the tools not knowing about it, and that its not still partially "present" somewhere, are very different things, as you discovered.
To be very clear, there was a tvheadend binary somewhere on your system when you thought there was none, and it was being started somehow via a mechanism you didn't understand.
Don't take any of that as criticism, it's just lessons learned.
