I suppose these abilities will make them useless for attack(since they'll not be attacking while casting), but good enough as meat shield on the front line. Taking that punishment.
There's also an alternative you could go for, since the NE have far too many casters to begin with.
Tomoraider in his campaign of Forsaken made a unit dread guard, this unit serves as the sentry mostly. He starts of with a meh health of 500-600 if I remember correctly and has an ability life drain [he drains enemies health over a (long)period of time, thus recovering his health as well, but renders him unable to attack so pretty useless to deal damage]. Now you could consider making elune's blessing something like this.
Also that unit has spell immunity but that would make a t1 unit too OP, unless, you strip their damage to equal a footman. I can see that working actually.
I'm fine with the Sentries not being able to deal damage while they are casting. Their cast is a late-game ability, and at a point where Sentries don't deal that much damage anyway.
Your description for a life steal ability makes sense, although I don't think it would be an appropriate ability for the Night Elves in general.