Glad you liked it!
For the memory leak I can't reproduce it (Firefox 102.0 on Linux). Is it always happening or only on some levels?
For the Witch-Doctor mind control, I could definitely reduce the number of uses and/or the range.
For the Mind-Flayer mind control, (or the charm abilities of some monsters), usually it feels "fair", it's only allowed to use the skills of your character that have an unlimited number of uses. Without this restriction it would do very nasty things likes bombs, potions, inspire, etc. With it, it's often restricted to main attack, and gives you the opportunity to counter it because they are delayed, (except for the Mercenary!).
For the main attack, I agree that the delay makes is risky (especially for AOE), and one needs to check possible counter-moves from the AI before a delayed attack. But it can be extremely efficient if always aimed at a target that won't be able to dodge it, either because it's on recovery, or because multiple characters are targeted at the same time. Maybe the un-upgraded damage is not impressive but it's usually easy to just target a monster on recovery (while other skills require the right conditions for AI friendly fire and traps). In my games, I used main attacks more than other skills, even more so when I invest upgrades in these skills.
For balancing characters, you are not the only one telling me that the Barbarian is very powerful, upgraded grapple with no recovery is maybe OP. I'm not sure yet how to balance it.
But for the Priest and Mercenary I'm actually very happy you liked them, I was feeling these one were underpowered, maybe we have different playstyles.
Thanks for these feedbacks!