Boss: Penitentiary Incarcerator (based on Brutus), map66dungeon

#1 I keep getting stunned/halted by the summoned monsters (when they are very many) - it may be a desync pulling me back to animate me taking the hit. I have unwavering stance so this is clearly a bug either way.

#2 Summoned monsters generate flask charges but not vaal charges

Summoned monsters don't give souls to vaal skills. They also don't give experience. It's designed that way. You can only use a vaal skill once on a boss. If you could use it repeatedly, it'd be way overpowered.

You might have to elaborate more on your #1. From your description it just sounds like you got blocked because you can't run through monsters. A quartz flask would help you if that's what you're trying to describe.