That's a shame it didn't work out with your friend. But, if she was willing to give Momo back after just one day, it's probably better she didn't keep her.
In terms of other things that may help:
In terms of other things that may help:
- as rubysmama suggested, Feliway might help (for me it didn't really do anything, but it also didn't hurt).
- There are also herbal/natural calming diffusers/pipettes that you can try (I had good experience with Beaphar No Stress, in the form of pipettes/collar; I already had Feliway Friends plugged in, so I didn't try the diffusers.)
- I've never heard of the vanilla extract method, but you mention that scent is not an issue for the cats so I'm personally a bit sceptical of its results.
- Definitely try some calming medicine for Momo (e.g., Fleur de Bach's Rescue Remedy, Zylkène or Vétoquinol...), as well as for the other cats if you think they are aggressive out of fear. If Prozac is the only thing available in your country, I'd definitely go that route.
- When you get to the pet gate stage and they are comfortable looking at each other, start to leave the door open for more and more time even outside of mealtime so they get used to seeing each other just "live their lives". Before you physically reintroduce them, they should be able to live calmly with the door open 24/7 (only with the pet gate in between them). You can get a pet camera to monitor them during these introductions.
- Try to designate a "neutral room" where both the cats and Momo spend several hours per day to get comfortable (separately), but which is a "no man's land". Then do the introductions there. Make sure there are plenty of hiding spaces and high-up places where Momo can go to hide (her running around scared is triggering the "cat and mouse game").
- As a last resort - this is NOT recommended by experts but I've done it - you can leash train Kaiya and have her on a leash during the next round of face-to-face introductions so you have a way to stop her from chasing/attacking Momo. But, be aware that if she is not perfectly comfortable on a leash this might stress her out more.
- Also, when you redo the introductions, it's good to not just let the cats see/meet each other during meal time. Basically, what this does is that it distracts them, but it does not change their underlying behavior. This method helps the cats to get to know each other under positive circumstances - but your cats already know each other.
So you need to not distract them, but train them to adopt a different behavior. You can watch some videos on positive reinforcement training. You can make your own behavior modification plan, but I'm proposing one below.
The goal is for the animal to choose the "right" behavior, which then gets rewarded. So, with your cats, you would actively reward whenever they choose to ignore each other.
Have a pouch with treats on you. Now that Momo is back in her room, whenever you see them come up to the door, sniff it and then turn away and walk away, say "leave it", throw them a treat and praise them. Then, you start opening the door with the baby gate. You open it for a very, very short time at first (maybe 3-5 seconds). The goal is to close the door before anyone has had a chance to get anxious or angry. If the cats come running at the sight of the door being open, you close it, say "leave it" and reward them when they walk away. Repeat again after 30-60 mins as many times as possible each day. Ideally, they should learn (1) to not come when the door is open, and (2) really cement the meaning of "leave it" into their brains.
Then, you can start leaving the door open for longer periods (1 minute, 2 minutes, 5 minutes...) as long as they are calm and show little to no interest in Momo. Always reward them when they choose to walk away and ignore her. At the same time, you do the same training with Momo, but you reward her for coming to the gate (because with her, the goal is different - you want to build her confidence!) Eventually, you should be able to come to a point where the door is open and the cats are ignoring each other for 30mins, 60mins, then the whole evening, then the full day... Ideally, you then start moving the baby gate, so they get used to seeing Momo in different rooms, but are always separated. And even when Momo is exploring "their room" or sleeping on "their" cat tree, they still can rely on the routine of "I ignore the intruder = I get a treat".
Once everything seems calm, I would introduce her to the calmest cat (which is Snow, I suppose?) and I would try to get her to bond with Snow. Once she's comfortable with Snow and they can spend several hours together (with ALL cats ignoring each other), then you can add another cat.
If this seems like a lot of work and a lot of steps - it is. You can of course modify the approach as you see fit, but I would strongly lean towards incorporating actual training into the reintroduction, rather than just trying to make positive association. In parallel, you can train your cats to do a whole bunch of tricks which would both stimulate their brains and build up their confidence so they aren't as scared/aggressive. Plus, many of the tricks can be helpful during the introduction process (e.g., sitting on a target, and then you slowly move the targets closer together).