This has been fixed in EiR, so that 1 man mortar units will retreat instead. However, the programming was reverted in a patch a while ago and won't be corrected again until 2.0 functionality is in.
Bring on 2.0! I wasn't around for that patch sadly.
I think 1 person can fire a mortar, though it may be troublesome. I think if 1 man can fine the MG then CoH should of made it the same for them all. Or at least force retreat or force pack up of mortars, and i guess AT guns and nebels could just be unable to fire. But it's so dumb that they just die, probably my biggest complaint with CoH period.