A) Yes it's random to some degree, its certainly never 1vs16, 2vs15 etc.

B) The power displayed is the power of the team you had up when rankings were finalized, not your max power

My guess is its something like the top 8 in a group will be randomly matched against the bottom 8. As someone who's in the upper half of a group for spacetime every season except the first, I can vouch that my first opponent has always been 9 or lower. After the first round I haven't paid enough attention to things to see if something similar is going on, i.e. if matches go according to seeding with no upsets then round 2 will be top 4 in a group versus 5-8 in some random matching, and so on.

Thus your motivation is to at least get enough points to crack top 8 in a group (which I guess would be top 64 overall) so that you can have an easier opponent in round 1.

Also this is still all guesswork so take it with a grain of salt, though the evidence does support it to some degree as I look over first round pairings in my spacetime.