In terms of seeding, it's based on previous results/perceived skill. In a pool of 16, there is 1 person who is basically guaranteed to get out (Boom, Isai, etc) 1 person who really should, and 2 more who should be really close to getting out. (And then another 4 who are at a lowe-B level, and the rest are unknowns or people who haven't had amazing results.)
At least that is supposed to happen but in terms of last apex and who seems to be registering now, there are more well known strong players than there are weaker/unknown players.
Like for last year, pool A was ridiculously difficult but then again all of them were pretty strong)
And yea,, doubles has a loser's bracket. It'll probably be bracket pools or be round robin pools (there is no way 64 singles will be round robin,) it really depends on size and number of consoles (especially overclocked)