I did a small test regarding the mortar pit and its resilience against small arms fire.
I always used 1 grenadier squad with 0 vet at short range (which is close enough to the pit, so the pit cannot defend itself)
See this picture for range reference
I will do a test with the cancer commander upgrade later. But for now, here are my findings:
44 sec to destroy
Unupgraded pit - no cancer commander, no brace
1 gren no vet, no lmg, short range
30 sec to destroy
Unupgraded pit - no cancer commander, no brace
1 gren no vet, has lmg, short range
60/65 sec (tested 2 time)
upgraded pit - no cancer commander, no brace
1 gren no vet, no lmg, short range
54/52 sec (tested 2 time)
upgraded pit - no cancer commander, no brace
1 gren no vet, has lmg, short range
50 sec to destroy
Unupgraded pit - no cancer commander, 1x brace
1 gren no vet, no lmg, short range
45 sec to destroy
Unupgraded pit - no cancer commander, 1x brace
1 gren no vet, has lmg, short range
1 min 37 sec to destroy
upgraded pit - no cancer commander, 2x brace
1 gren no vet, no lmg, short range
1 min 6 sec
upgraded pit - no cancer commander, 1x brace (could only activate it once, since it got destroyed just as the cooldown was wearing off)
1 gren no vet, has lmg, short range