Well, here's an attempt to answer some of your questions. Unfortunately, the aircraft is too new, and very few (including me) have had enough actual BIS time to make some subjective conclusions as to what is "best".

But here are the "facts", for what they're worth:

Coach Cabin is rows 5-20, Seats AB and CD on either side.

5AB are bulkhead, and designated Disability Seating.
5CD are bulkhead, and open to anyone at anytime
6ABCD and 7AD are Select Seating for Elites and Full Fare Pax.
7BC are Coach Choice.
8ABCD are designated Disability Seating.
9AB are Coach Choice.
12ABCD and 13ABCD are exit row seats. Of these, 13AB are held for Coach Choice, the rest are available to Elites at any time. One might assume that Row 12 has limited or no recline
19CD and 20ABCD are blocked, and held for assignment at the Gate. These are for Unaccompanied Minors, and for seating families together when they could not get seats together on their own.

Everything else is open to anyone, at anytime,

So, the totals for the CRJ900 are....

6 seats held for disabled passengers
12 seats for Elites (Select Seating) including 6 exit row seats
6 seats for Coach Choice, including 2 exit row seats
6 seats held for assignment at the gate only
34 seats open to anyone at anytime

Grand total: 64

Hope this helps a little.