No progress...now what?

2.5 weeks into new cat intro and we've hit a stalemate, having made no progress in the last week despite our best efforts. Resident cat (Kaylee, 2yrs) is usually happy and carefree all day, until she sees the kitten (Zoe, 12wks). The first time they see each other after having been apart for even a minute or two, Kaylee hisses and growls.

We've been doing supervised contact with a Feliway diffuser in the room. When they first see each other, Kaylee will hiss of course, then after a few minutes, she lets her guard down and seems content to be in there. For a while, they'll both seem fine, leaving each other alone, or letting us play with both of them. Then out of nowhere, Kaylee will remember she hates Zoe, hiss at her, and ask to leave the room. Zoe has an insane desire to constantly hunt Kaylee whenever they're together unless we keep her occupied with other toys. If she manages to sneak up on unsuspecting Kaylee, a loud brawl breaks out and we separate them. After a time out, we start over, and it goes exactly the same.

It feels like we've done a hundred of these little meetings in the last week and every one is exactly the same. I don't know what else to do....

*interesting note, as I'm typing this, Zoe is in her kennel across the hall in the bedroom. Kaylee was in there and everything was peachy, then she came in here to visit me for less than 5 minutes, went back into the bedroom, and started hissing away.

Everything you describe sounds very normal.
Females are usually less accepting of new comers than males.
They may never like each other, but it sounds like you have a strong foundation for two girls who will co-exist well.

As long as there is only hissing/growling I would simply move forward at this point and leave the kitten free to roam and let them work things out.

Pay attention to your resident cat first in everything (play, petting, feeding etc).

And watch to make sure the kitten does not set up a pattern of harassing the cat, or vice versa.

Food has always worked in my house when I've had to introduce/reintroduce. Feed them in the same room, starting far away then gradually move them closer if things go smoothly. If the hisses start again, move back and try again. The goal is to establish an assocation with good things (food) with the other cat.
Good luck!

It sounds to me like things are going fine. Some cats are more accepting of newcomers than others. The energy of a kitten is bound to be annoying to an adult cat that isn't wild about having a strange cat around. As long as they aren't trying to rip each other to shreds, they're fine.

Keep telling Kaylee she's number 1, always give her toys and food first, and tell her the kitten is for her.

I had a fair amount of luck with two adults by feeding them in close proximity to each other, and having treats ready for when they were quiet near each other. You know, so they got the idea that good things will happen when they're 'nice'. I also had toys around to distract them if they started staring at each other. Good luck!