Rarely but occasionally, the cups daemon fails to start properly upon reboot. I can restart it from the command line, but is there a way to restart cups from the localhost:631 web interface? I've search the webpage and the Internet, but cannot find a way to do this.

No.

It is cupsd itself that LISTENs on port 631 so if cupsd didn't start there isn't a way to start it via that port:

Running "lsof -i :631" from command line shows (at the beginning) it the 631 (ipp) port LISTENers for tcp and the udp port as well:
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
cupsd 23664 root 7u IPv4 216810900 0t0 TCP *:ipp (LISTEN)
cupsd 23664 root 8u IPv6 216810901 0t0 TCP *:ipp (LISTEN)
cupsd 23664 root 10u IPv4 216810904 0t0 UDP *:ipp

You might be able to add CUPS to WebMin and use that tool to start cups instead. A quick web search reveals this page among others:
https://doxfer.webmin.com/Webmin/Printer_Administration

That makes sense. I should have thought that through. THX. I'll check out webmin.

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