Comfort Zone Multicat Refills

Q: Do comfortzone refill fit in the feliway difusers?

90% Sure yes. I've been using Comfort Zone & Feliway interchangeably, just what I can get ahold of, and haven't had a diffuser fit problem.

Q: Does the 2x refill work with the original diffuser, I see the outer casing is different, is that needed for the 2x to be effective?

Yes, they are backward compatible.

Q: Do all contain a diffuser?

No, there is no diffuser in this pack. You will see a picture of the diffuser on the front of the package of those that contain a diffuser(s).

Q: How well does it work for indoor cats marking because outdoor strays are driving them crazy?

I've not used these specifically in that situation, though I know exactly how aggravating that is! I've introduced several rescued feral cats into my main group of indoor cats in years past, and only wish I would have had this product back then. I can't say it will fix the problem you're having, but if it were me, I would sure TRY it. I'd saturate the home with the plug-in diffusers, then spray directly on whatever areas your cats are in when they see/hear the outdoor cats. I honestly think it would be worth a try. Right now I have an outdoor kitty who comes and lies up against the door here (I take care of her, feed her, and she has shelter and a heated bed!), but my indoor cats here just lie against the door on the INSIDE when they hear her. No meowing, no crying, no scratching. I'm in a small apartment and two plug-in diffusers seem to work wonders. I personally hope to never be without this product... even my petsitters always remark on how they "can't believe how well everyone in here gets along". Good luck.