Did the patch itself get wrote over with a lower volume setting?
Is the new song using the exact same patch as the original??

"Is there a setting option somewhere in RealBand that would impact volume level on an individual channel that would "stick" over system restarts? "

This is possible if you open the program using a template ($Options.seq) or such, but highly unlikely.. if you were using it you would know about it.

"Tried increasing to 127 in the event view, no significant change to sound level. Tried applying a 200% velocity increase via edit menu, no significant change to sound level."

This only changes the velocity (how hard you hit the key), not the overall volume control for the MIDI channel. See if there is CC #& or 11 causing the issue in the event list for channel 1.
My first hunch was a patch got saved with a lower volume applied...

I do not work here, but the benefits are still awesome
Make your sound your own!