From the description; the .deb package install requires sudo (root/admin) rights to be installed. It's not a backdoor password, but since it can be used to gain root rights on the host it's a credential to protect - hence the security best-practice advice of not using the same password for the admin user in TVH.
Yes - definitely.
When you install tvheadend from a deb or a repo (which adds it to your system services so that it runs on boot etc.), rather than just running it as code from the command line, you can't run it with the 'no access control' flag (which lets you in without a login and password). As a result TV Headend, when installed from a repo or deb, requires you to create an initial admin user, so that after installation you can actually log in to the web interface for user setup, tuner config etc. and to add other users. I believe this may be the 'superuser' you are talking about.
This admin account definitely shouldn't be the same as your login and password for Linux.
Once you are in the web interface you can create individual user accounts with their own logins and passwords (and different rights levels, including admins), including additional accounts with admin rights, and you can create an anonymous user with very basic streaming rights (and no web interface rights) that won't require a login and password ( as it gets boring typing in logins and passwords into VLC when you just want to open a stream)
Separately AIUI there is a superuser file you can create that will let you create a new admin user if you get locked out from TV Headend's web interface (because you've forgotten, or deleted, all the accounts with web interface access for instance). I believe this is referred to as 'the backdoor'. This is stored in :
/home/hts/.hts/tvheadend/superuser
It's only for use if you get locked out AIUI. I've never had to use it. It only works with the English language UI I think.
You can also reset the password, if you installed via deb, by reconfiguring using dpkg-reconfigure I think.
so you support my statement on WTF is the backdoor password. Its a clusterfrack that seriously needs updated docs and thos docs need to conform to what is in the installer and web UI
AIUI development on tvheadend has slowed down quite a lot over the years, as there has been less and less functionality to add, and a lot of the earlier key developers have had less time to devote to the project as their lives have moved on. Installation via a deb package is just one way of installing TV Headend, and is specific to Debian/Ubuntu flavours of Linux. There are numerous other ways of installing and using TV Headend (Docker containers, building from GitHub and running from the command line etc.)
It's an open source project, like Kodi, developed by volunteers largely in their spare time. Like a lot of projects, time has been spent more on developing the software than writing up the documents. A lot of TV Headend is pretty self-explanatory if you understand basic DVB and IPTV - and the forums usually contain a lot of answers to questions that have come up.
I like it, I use it daily, it solves problems for me. There are other solutions such as MythTV, VDR, NextPVR, TVMosaic etc. I don't find them as useful or as easy to use (VDR had no decent support for UK DVB-T2 Huffman EPG compression for instance)
If you don't like TV Headend then nobody is forcing you to use it! If you think the documentation needs to be improved, nothing is stopping you improving it yourself.
You asked for a solution that would allow you to create a PVR back-end with both ATSC 8VSB and IPTV streams. TV Headend is what a lot of us would use to do that. If it's not for you, it's not for you.