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1. For now I am starting qjackctl in a terminal window. In that window I get the following warning:
Warning: no locale found: /usr/local/share/locale/qjackctl_en_US.qm
Since I can see all the text in English on the gui widgets and the message window, I am guessing this warning is not important.

You guessed it right. It is not important and it's just a warning. It only gets supressed when a specific translation file is found for your current LANG setting. As en_US is in deed the native one, you're fine. You rather ignore it and carry on ;)

2. In its message window, qjackctl complained about a missing artshell. I don't know if this is an important problem. In any case I found the option to enable/disable the execution of a script at start time. I unchecked that option, and now I don't see this complaint anymore.

Again, you've done the right thing. The artsshell startup entry is a legacy from early times, when KDE aRTs sound service was ruling and annoyingly grabbing the sound device in exclusive mode. If you don't run KDE or aRTs is not your desktop sound service, then you may forget or even ignore this issue.

3. In its message window, qjackctl issued a complaint mentioning FIFO and permissions. I looked around the web and I found the suggestion to add these lines to /etc/security/limits.conf:
edward - rtprio 95
edward - memlock 512000
edward - nice -19
After restarting my computer (I'm sure there is a better way), this made the error message go away. From what I read, I have the vague idea that these settings have to do with allowing user 'edward' to execute processes that request "real time" operations. I won't ask you to explain each setting, but I am curious to know whether you think this modification of /etc/security/limits.conf is a reasonable thing to do.

It is not just reasonable but rather necessary. These settings allows you (edward) to run programs which needs real-time scheduling privileges to behave as designed. This is the case when you start jackd in real-time mode (-R) and it's the recommended way of doing things. So you can chalk this one right too.

4. I compiled and installed fluidsynth and qsynth from their sources. Then I started qjackctl and clicked the start button; and this appeared to start up jack successfully. Unfortunately when I attempted to start qsynth it reported the following in its message window:
22:10:04.050 Qsynth1: Creating synthesizer engine...
22:10:05.080 Qsynth1: Creating audio driver (jack)...
22:10:05.081 Qsynth1: Failed to create the audio driver (jack). Cannot continue without it.
fluidsynth: error: Couldn't find the requested audio driver: jack
So I am stuck at this point. Any idea what could be wrong?

It looks like fluidsynth is failing to connect to jack as a client. This might be a symptom of shared library clash, meaning that it might be probable that you have different versions of the same library.so under different paths. For example, maybe you have libjack.so, or libfluidsynth.so under /usr/local/lib as installed from the sources by default, and also under /usr/lib but installed from your distro packaging. Check that out and try uninstalling the distro packaged ones first and try again.

This is just a guess of course. You should try whether the same issue appears when starting some other jack clients for instance. Example clients which are usually installed with jack itself are: jack_lsp, jack_metro, jack_simple_client, jackrec, jack_showtime, etc. Try any of those and check if they run alright.

Byee
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rncbc aka Rui Nuno Capela