Okay, so I have never used Myth (yet) but did read something about it
a few months ago. So don't count me as an expect, & forgive me if you
know this bit...

Myth needs both a "back end" and a "front end" to work. These can be
on the same PC or separate ones. The back-end runs a database, does
the recording, etc. The front-end then connects to the back-end and
runs the presentation layer to show the recordings & user interface.
The idea is you can have a network with one BE & several FE if you want.

The messages "No UPNP backends," "Unable to login to database," etc.
sound as if you only have a FE & no BE. Alternatively you could have
connectivity issues from FE to BE. Maybe the BE is crashing, not
running, etc.

Install Mythbuntu. MythTV is notoriously hard to configure. If you install the
Mythbuntu metapackage and then run Mythbuntu from the session manager most of
the hard work is done for you.

I've downloaded the Mythbuntu ISO image, and will see about installing
it. There is one question I'd like to solve first, though: I would
like to keep the Ubuntu Studio setup, since it has a number of
music-related packages I like. When I went to the Mythbuntu website,
I found a link that supposedly would allow me to install Mythbuntu
without losing what I already have in my Ubuntu partition(A so-called
"metapackage" is involved.). HOWEVER--every time I click on that
link, Firefox gives me an error message saying that it doesn't know
how to open the page (which has a "php" suffix), because the protocol
(apt) isn't associated with any program. How do I correct this?

I would not work from the ISO image. This will require you to install it as a
separate distro instead of as a desktop. All you need to do is install it from
Synaptic and it will appear in the sessions menu on the login screen. This way
it will use the same root partition and home folder.

Load Synaptic and go to Meta Packages (multiverse) and install
mythbuntu-desktop. Same goes for Ubuntu Studio, except it is in Meta Packages
(universe) and install ubuntustudio-desktop. Doing it this way you will avoid
all sorts of potential problems.

Systm.org has video on installation of Mythbox

Unfortunately, that episode is just over 3 years old, and is therefore
at least somewhat outdated. Also, it does not go into the database
setup issues I am currently grappling with. Thanks for the
suggestion, though!