Reboots is how I kept MLAT running for the past year when it randomly stopped, but no amount of reboots or "sudo systemctl restart rbfeeder" would fix it when MLAT quit about a month ago. I had an error message when running "sudo systemctl status rbfeeder" to the effect:
"File "/usr/bin/mlat-client", line 108, in <module>
adsbexchange rbfeeder[642]: open(pidfile, 'w').write(pid)
adsbexchange rbfeeder[642]: PermissionError: [Errno 13] Permission denied: '/run/mlat-client.pid'
This error cleared when I applied the patches in my post above and MLAT then started working. One can now restart the process with "sudo systemctl restart rbfeeder" and "sudo systemctl restart mlat-client", MLAT now happily restarts without a reboot.