Better late than never. I will add my opinions.
The rule 20.5 (p17) under Characteristics states:
"Garrisons may not move or attack, they may only defend. Garrisons do not participate in combat if stacked with other units."
If garrisons are stacked with other units:
a) Is "in combat" meaning they do not participate in "attack combat" or "defensive combat" or both?
b) In whatever combat they do not participate, are they ignoring the combat results also?
Not necessarily, here is an excerpt from 11.5
NOTE: If a disorder result causes all defending unit(s) to be
eliminated, eliminate any garrison units in the defender’s hex.