Along with the guilt you feel is a ton of frustration that no matter what you tried, nothing helped that lovely girl.

Many people will adopt an only cat - and your have given her future owner a wonderful opportunity, and one for Katie as well!

Hugs yourself, you tried more than most people would have.

Ember sounds like a great four-month-old kitten! I am glad he and Kyubey are on their way to being buddies already.