Cool concept, I agree that there's balance issues. Movement and platforming and stuff felt good, and I like the new assets.
It was a struggle to stay alive, let alone build enough mana to activate the fountain, and that was after four or so tries.
One of the biggest issues for me was that if you threw a seed too close to another seed, or near the fountain, it'd just disappear. It'd be nice if they tried to seek out the nearest possible tile or returned to your hand or something, or at least gave back the mana used to make it.
There's a few things that could make the game a bit easier to play. You could start the player off with more mana. Or you could move the seeds a bit closer together. You could also slow down the corruption rate.