Hi Viper,
Seems we are in different time zones, which slows down communication. I believe I can help you, but in that case I need you to follow my recommendations and not to mix in (outdated) stuff that you find elsewhere. I'm pretty sure your premissions are messed up already. No use it making it worse.
So, I'd try this:
(1) Again, please uninstall whatever tvheadend version you have installed in the meantime. If you still have some kind of tvheadend-related users on your system, which you can see, please remove them as well. Then reboot your NAS.
(2) Download the TVHeadend
4.2.7-14 package matching your NAS platform from the link here:
https://synocommunity.com/package/tvheadend
Do
not install anything from another source, please. If you are not sure which architecture / package matches your NAS, you can look the architecture up here:
https://github.com/SynoCommunity/spksrc/wiki/Architecture-per-Synology-model
(3) The package should install. When it asks you for user name and password, just keep the default values (admin, admin) for now.
(4) If the package installs, run it and try to log in with admin / admin. If it does work, be happy, continue with the standard configuration and don't forget to change your password later on.
(5) If it does not work, report what the error is here.
(6) Then we need diagnostics. For that:
(a) Stop the TVH package from running in the DSM package manager.
(b) Log into your NAS via ssh as admin (using your root password)
(c) Gain root priviledges via
sudo su - (using the same root password again)
(d) Execute
cd /usr/local/tvheadend/var via your ssh terminal. Then...
(e) Execute
ls -la /var/packages/tvheadend/target and report the output here.
(f) Execute
synoacltool -get "/var/packages/tvheadend/target" and report the output here.
(g) Execute
cat tvheadend_install.log and report the output here (use pastebin or something, if the file is longer than a couple of lines).
(h) Execute
cat tvheadend.log and report the output here (or via pastebin).
(i) Some of the files might not be found / not exist. No problem, just let me know.
That should give me a pretty good idea what's wrong. Don't do anything else, please. Just report and leave tvheadend alone for the time being...