Please excuse my constant comments, lol. I also can get somewhat long winded.
I completely understand that you have to keep game balance. But, with narrative control of the story, you can use that in ways to dampen her combat prowess. If she's turned half-turned, then that's easy, she's weaker because she's used to being a killing machine and she needs to get used to her weakened state. If she's full delirious then maybe she doesn't want to join the player until she's sure he can defend himself from her if she loses control. It's already alluded to that she's stalking the player when he goes out anyways because she's always there to bring you back if you lose. If you don't want to deprive the player of companions, you could add mercenaries to the trader area.
But this is just chit-chat, no need to think seriously about any of this. I'm just giving un-asked for advice afterall. Thank you for your diligence.
Fully understand, and yeah her hybrid form is already weaker than her Delirious form, the amount of how much weaker she'll be will probably increase soon because at the rate the poll is going over on my patreon, bulk of people don't seem to mind if I take Feb to really get to the backlog of mechanics which will include companions also having their own stats to level + equipment.
Something I HAD been toying with... is yes fully restoring her killing ability in her 2nd state BUT she needs rest after the bloodshed as to not give into the Delirium completely... so it could be a thing where you bring her out in her Delirium forms at key points to do maximum damage/for extremely difficult upcoming fights.
The other companions won't get this kind of cooldown, they may have something to them, but nothing to that degree. There very much is a story reason as to why she'll need it... but... That's VERY big spoilers

And then of course, Her in her Hybrid form will have a much lesser condition involved. I.e. killing has a chance to either up her corruption by a little or in some cases.. might jump her corruption up significantly depending on the type of enemy.
And as for mercenaries... lol already planned for Act II. Already have several scenes/locations set up where you'll be able to hire mercenaries from different factions. Some mercenaries will have perma death if they have side quests attached to them, but the bulk will be standard to those factions.
If you've ever wondered what those different currencies are in your inventory... Trader Tokens, Crimson Coins, Delirium Ducats.... Yup. I put that there from the start as an extremely early tease to confuse people up until it began to make sense as those and others will be the currencies used for those different factions.
If you look at what used to be the first image on the main menu, you'll be able to spot 2 of the story mercenaries fighting behind the mc and Ophelia
~Cheers!