should be fine, they get a new save every time they take damage at a +4. So that's +16, +9, +10, +7, Pretty good chances of most of them saving after two rounds.
You can always lower the save DC, or give them a special boost (potion, magic fountain, magic beans or berries) that give them a save bonus, or just Will saves, or even just hallucinations or whatever.
Or maybe increase the save bonus when they take damage, like a bonus equal to the damage or half the damage or something?
Usually, you can't attack with a limb you are grappling with, but sometimes you can hit your enemy with your other enemy.
Just choose whatever is the most fun.
That does make it seem better, though it concerns me that it could kill most of the party with a successful full attack.
Also, if it grapples with one if its 3 vines, can it still use that vine to attack with?
Don't full attack them then, spread the attacks out over all the party. and no, you cant attack with a limb you're using to grapple.
Dead dead, or mostly dead?
Dead until they get taken to a city to be raised again, dead. Ha.
I'm guessing there were tactical lessons learned. Probably they are learning the value of knowledge checks, buffs, positioning, and supporting one another. If your players get something out of the encounter that teaches them how to handle other encounters, it's still a win.
