does SCH gets better aoe heals at higher level?

[Rongway]

Can you get knocked out of casting? I suppose that might be a once-in-a-blue-moon case for {Surecast}.

Damage over a certain percentage of your health can interrupt you. You don't have to Surecast a pre-casting Medica/Helios in most cases, but yes, for certain attacks (Primal ultimates at-level) you'll need to Surecast.