Easier summon

No? I don't think you have an easier/harder time summoning units you've pulled already - there's no correlation.

...not particularly. It's mostly a matter of luck. But if you're really interested, your best bet is likely to save up tons and tons of orbs (and feathers, obviously) so you can snipe on Blues. It's what I'm planning to do with Libra, if he gets demoted.

The odds are the same so it really comes down to luck. I didn't pull a single Nowi in my first year of playing...and have pulled five or six since. Alternatively, I got my Gwendolyn to +9 and then suddenly stopped pulling the up until then super common unit.

Took me 3 months to find the last 2 Eirika, so yeah, long road ahead of you