Mistform isn't stealth so they could still use it just fine, and from the wording it sounds like the damage after the duration ends might take them out of it, but don't quote me on that.
If you're asking if Mistform will mitigate the damage... probably no. Mistform only mitigates projectiles and doesn't mitigate melee, damage over time, and area of effect damage unless you're hitting someone in Elusive Mist where they get AoE damage mitigation.
And just because the damage of a projectile is mitigated 100% doesn't mean the additional effects are.
Nightblade's canstill use Shadowy Disguise while PoL is on them, but the damage from the explosion will pull them out.
Most of the time yes, but it is possible to cast Shadowy in the same millisecond PoL goes off so it forces you back into cloak and you skip the reveal. It works the same when I have Sorc curse on me. Not something I can do regularly but it does happen once in a while.