So if you don't let any drill get destroy and you do them in order then it will have a relationship =p ( but i'm just trolling here) the main reason to take the letter of the drill is to check if there for missing data. for example in the heat of the action I forgot to write down the first B drill. and i didn't see that until the end once i check before posting.
forgot to sign up. and for the fact that the reward are the same as survival. well not for Cambria. they should be on the T1 reward list, but it seems consistent with the T2 reward (for survival). so i hope it's the same as the survival and Cambria is just a mistake devside. ( less work to do =p )
Rewards follow survival rotations for completed extractions. Destroyed extractors pause the rewards instead of skipping. So 2 'a' rewards, a 'b' reward, and a 'c reward, then repeat. Enemy level progression seems to also go by extractors activated, not by time elapsed. so if you take 15 minutes avoiding extractors, enemies will still be same level, but if you spend 15 minutes constantly rolling through extractors, enemy levels will have gone up signifigantly.