Can't speak for the others, but Bramble definately
a) Sticks on after he leaves mist if you cast before he enters it.
b ) Is not applied if already in Mist form, but mana is lost and skill goes on cooldown.
I know for a fact Frogboy has experienced firsthand (and died because of), so maybe it has a higher than normal chance of getting fixed
I see two solutions, either cancel spells with erebus as target when he goes to mistform (best solution imo), or allow friendly skills to target him while in Mist, in which case he should also be targetable by opponents if detection is present.