Yes, thank you to both of you - sorry for the delayed response - I was hoping to have better news to report, however -
Still getting my butt kicked here. Overnight my WiFi router must've done an update, as it reset itself to factory defaults - which were not in the Pi.. SO I unplugged the pi, took it to a monitor and keyboard, got it back online, took it to the antenna setup and plugged it back in... says it's back online again (but, of course, not reading any airplanes).
Attempting to self-help has led me to more frustration than success.
I will share what I've found is true about my system in hopes that my mistake is evident. In the .ini (/etc/rbfeeder.ini) file, if I comment out the lines below MLAT and UAT, as in many working systems, the system remains offline. If I stop rbfeeder and run rtl_test -t, both dongles are still found, still serialized as before - looks like nothing has changed there. If I remove the comment hashtag from the lines I referred to, the system appears to start, but is unable to read the attached dongles. I have network mode set to "false" to indicate it should read the dongles it finds onboard, not a remote feed - I believe that's how the instructions read?
I do see that there is no MLAT client installed - as shown by the test above with my results here:
pi@raspi1:~ $ apt-cache policy mlat-client
mlat-client:
Installed: (none)
Candidate: 1:0.2.13+deb11
Version table:
1:0.2.13+deb11 500
500
https://apt.rb24.com bullseye/main arm64 Packages