I would say you are interrupted only if the critical renders you unable to continue. E.g. Stun. If it just inflicts hits, and you are still conscious, you can continue. The Mentalism Companion adds a Spell Concentration skill, and optional rules about breaking concentration. You make the maneuver at a penalty of -20 for a hit and -10 to -50 for a critical (A through E).
In RMU, you would make a Mental Focus maneuver to continue concentrating if you are stunned. Concentration is not otherwise broken by being hit.
For #2, that's correct, I think it's clarified in an errata somewhere. It's explicit that when rolling on the base spell attack table, you use the realm that is most advantageous to the target (RMSR pg 99).