I totally understand why you have had to make this decision and you certainly can not live in a house with one cat attacking another.

Just a suggestion, have you considered inviting a behaviourist over to your home to see if there are any training methods that could be used to modify Cirrus's behaviour. (Like Laura's Babies used with her Samantha.)

I could find a small donation towards a behaviourist visit, if you decided to go this route and this might be easier than trying to adopt Cirrus out during kitten season and if it didn't work out, Cirrus should have an quicker time in autumn finding his new home.

All the best