Companions can rest, even if they have full health, however don't forget only one survivor can rest per round, which includes companions!
If the SPECIAL ABILITY side of the die is rolled, unless it's during a round of combat that the enemy will be defeated in, the ability automatically activates.
During combat, you are able to decide which die does what damage. For example, if one survivor and the defecting guard both roll cunning, but there is only 1 cunning chapter dice, you are able to choose the defecting guards cunning so that the enemy will do no damage to anyone.

The Defecting Guards CHARACTERISTIC does work against the boss!

If they pull plague, follow the usual rules for plague as if they were a normal survivor, following the same thresholds.