Curiouser and curiouser ;)
Case 1 (as per Makanyane's saved game)
1. 1 garrison unit present
2. Mercs available
= NO CTD (merc garrison)
Case 2
1. main army and garrison position swapped
2. Mercs available
= CTD (n/a)
Case 3
1. main army and garrison position still swapped
2. Mercs not available (i.e. hired and in settlement or in region)
= NO CTD (empty garrison)
What does this tell us?
EDIT: If your tests hold true (mine never seem a probability of 5 out of 6 or whatever - seems black and white) then perhaps the *chance* of something going wrong increases per unit. Yes, I think you will get more CTDs in your mod than you first anticipated. :(