On second glance, the memory leak just seems to be very situational garbage collection. The RAM usage goes up to about 1.5 GB over the course of a dungeon, but apparently gets cleared if RAM runs out. Plus it's definitely cleared at the end of the dungeon. So all good there.
For everything else, I'll try my best to give some productive playtest feedback:
Witch Doctor's mind control ability should definitely be limited in the amount of uses. Everything else feels fine about it. I've noticed that about 3 uses is the break point where you actually run out sometimes, but not all the time. However 4 uses makes it hard to run out and 1 use would make the ability worthless to upgrade. So my recommendation would be to keep 2 uses, and instead of increasing the amount of uses, that upgrade could increase the range (since range is not that strong if you only have 2 uses anyway).
The Priest's extra turn eliminates Recovery, that alone makes it an incredible support ability. You can even chain it on consecutive turns (if you skip your Priest's extra turn), basically eliminating its downsides. The rest of the Priest kit is very meh though. The lightning attack is by far the coolest attack if it hits, but sucks in practice since it's just too easy for the enemy to make it hit yourself.
Mercenary's secondary abilities are meh. The bomb is just a basic attack with limited uses and the net is an average CC option. But its main attack is the gold standard if you ask me. Would be cool to see other characters with a reliable primary.
Barbarian is probably the strongest character in the game. A limited use instant 2-damage attack is better than most primaries. It should probably be more limited in range & uses. The grapple is great, since you can move enemies 2 tiles in an unorthodox way, but not broken. The part that makes it really strong is eliminating recovery. Here's a suggestion for an alternative: whenever the Barbarian kills a unit, it heals 1hp.
As for the base attacks: maybe I'm simply too obsessed with them because I try to take as little damage as possible and they become a lot better if you accept that you'll take damage here and there. I definitely like the underlying tactical idea though and once there's only 2-3 enemies left, they're pretty enjoyable. More enemies means that there's always someone in range to thwart you, with multiple abilities that can mess you up in different ways. Especially the AOE attacks suffer since it's actually really easy for the enemy to make them hit yourself. On most characters it also generally feels like I'm just 1 upgrade down to make good use of them. Either the damage, or the movespeed, or the utility of the attack is lacking, and that's also while not having upgraded any of the secondary abilities.
I'll hopefully get to play the game a bit more some of these days, and maybe get to pay some attention to how strong each character's primary really is when making best use of it.