The Timer0 is random.
a) So you have your range, but you should only use the most common ones. We have a range over twelfe different ones and we only search in three common Timer0s of ours. You probably have two or three common Timer0s; when they are all equally common, then use all of them.
b)Yeah, is is usual in BW2 that is may take a few attempt to get your correct Timer0. In BW1, you usually only have two Timer0, one of them very common and the other one negligible - which means you practically always get your mon in the first attempt and it almost never takes more than two.
And breeding RNG in BW2 is impossible, and catching is a chore, so we only do Hidden Grotto and legendaries in BW2 (and Gift Pokemon).
b) The more the better; the only purpose is to find the most common Timer0s (plural).