Hey, are you still having problems? I have been using Tvheadend for a while now and maybe I could help. First thing, please make sure you have Expert settings showing in Tvheadend web interface. This is under Configuration, General, Base, fourth line says "User interface level:", in the drop down box it should be "Expert". Once you make sure this is selected then let's see if I can help you.
First, for the login loops, this is most likely to related to the user access tab. There should be a superuser (master admin user). I think I created mine when building/compiling Tvheadend or maybe I created one during the initial setup. I can't remember but this user does not show up in the list of users so you will not see its username. Anyway, you should always be able to log in with this username and password, no matter what the user access tab has listed in terms of users. This username and password is located in /tvheadend/superuser folder on your LibreELEC machine (check in .kodi/addons/service.tvheadend42 or something like that).
Second, for the login loops, once you log in with the superuser, make sure your tvhadmin user is correctly set up with permissions and allowed IP addresses. I can send you screenshots of my settings if you are still having problems. Make sure Enable, Admin, Web interface, are all checked in the admin user Edit Access Entry dialog.