I think that the card will still be useless after receiving a buff like this. If we look at other T3 units with CC abilities, those abilities are always very aggressively costed when compared to spells, due to their more restrictive nature (in terms of positioning and bound power). Examples being Fathom Lord, which has excellent stats, and a cheap paralyze (the strongest kind of CC); Swamp Drake, which has solid stats for a flying unit and what is essentially Oink for 30 power (with 2 fewer seconds to hit the affected unit without reverting CC); and Timeless One which is cheap and has a dirt cheap ability.
Twilight Hag, on the other hand, has abysmal combat stats, and even at 50 power, the ability is still worse than Oink in most cases. A few things to consider:
You have to bind 75 power in order to use Twilight Hag's ability.
The unit is occupied for the full duration of the ability, meaning that she doesn't do damage or knockback while using it.
The ability only takes effect around the unit, unlike Swamp Drake and Timeless One where you have a decent bit of range to place the CC optimally. Combined with her low HP, this makes getting her into position very difficult.
If the Hag is CC'ed or teleported the ability is cancelled. This means that the ability is completely countered by several spells that don't cost that much more (even after your proposed change) and are very likely to get more value by hitting other enemies.
As a non-squad M unit, she can be completely locked down by knockback.
The ability radius is actually much smaller than the aura would suggest (small enough for ranged units to hit her, charge units to charge at her, and Grigori to disintegrate her), so there is room to increase the AoE.
The ability doesn't affect female enemies, as well as many beasts and some other units (mostly anything that doesn't have a male voice). This may not be very significant right now, but as weak cards are buffed, many of those unused cards will be used and this downside will become more apparent.
As far as the unit's combat abilities go, it's not even cost effective against units it should counter (e.g. Fathom Lord, Virtuoso, or even Fallen Skyelf).
And the upsides:
She can't be knocked back while using her ability.
Her damage buff/healing is nice, but only really does much if you have a lot of units around her (which makes her even more vulnerable to CC).
Her knockback radius is pretty big, making her good for locking down M and S units.
I can understand wanting to be conservative with buffs in order to not break things, but I think that if we're going to make much progress we can't be afraid to be more aggressive with our buffs. We should be putting units into a state that puts them on par with other good units; then if they end up being too strong, we can always dial them back after testing them on the test server. A common and effective methodology for pre-release balance (which this is) is to push the unit to what you see as the reasonable limit (some designers would argue even beyond that limit) in order to find the unit's strengths and to get a good feel for its interactions, and then dial it back if necessary.
My proposal:
I think that buffing the ability and increasing the unit's HP for more survivability is the right approach, it just needs a LOT more:
Increase HP from 785 to 1350.
Decrease ability cost to 15.
Increase ability radius by 5m.
If CC hits more than 1 unit, it's still generally a cost effective way to counter the Hag's ability. Pure fire has Wildfire, which easily kills her even at 1350 HP unless you cancel the ability and move her (and pure fire doesn't need any help in T3 anyway). This is a huge HP buff, but it still doesn't put her in a normal stat per cost range for a T3 unit (Silverwinds have nearly double her current stats).
A better alternative would be to give her damage resistance and CC immunity while using her ability, along with a moderate HP boost and ability cost reduction; but that would probably be a lot harder to implement - the above solution should suffice.
