To answer your first question, as long as there is availability in A you will be fine.
As for your last question, expect they will treat both of the segments as part of the upgrade, so no two seperate upgrades.

If you fall short of the miles required, you could always buy up 25,000 miles.

Also for holding an upgrade, my last dealing with a similar matter was in Oct last year. I had to create a reservation (a whole new one actually) to secure a seat in F (flight was zeroed out 3 months in advance, and a F1 appeared on a Sunday morning). Oh and they could only hold it for 3 days, but they extended it another 2 days until the matter was solved (by moving my original paid booking to the new dummy PNR).

Good luck