Well - after 4 full weeks of trying and trying we had to make the difficult decision to let Max go back to her previous home.
This was just not going to work out. Max settled in here with no issues for herself - found the food to her liking, improved litter box habits, no howling at the door despite being free roam at her previous home, etc. Unfortunately my other two, Yeti and Sparks, didn't have such an easy go of it. The more comfortable Max got being here, the more aggressive and dominant she got. The other two just did not defend their positions in the household and were consequently being terrorized by the end of last week. Max was getting pretty vicious with her attacks and there were some very minor injuries - scratches and bald patches - and some pretty serious psychological effects. Nothing I was able to do could distract her once she decided she wanted some action and we had a couple of sleepless nights - me in the bedroom with the originals and the husband on the couch with Max. Yeti and Sparks took to the underside of the bed and would not come out for love, money, extra special "I'll do magic tricks for them" treats - nothing. I worked pretty closely (daily) with the vet for the last week and nothing was going to persuade Max to let up on the aggression so he suggested two final options: anti-anxiety meds for all of them (possibly on a permanent basis
) - or send Max home.
I'm not prepared to dose any animal with any medication unnecessarily so we made the call and Max was happily welcomed back. She basically marched back in like she'd never been away! My friend is reporting however that Max seems to be slightly better behaved having spent a month away! She's convinced Max has learned a lesson and knows she may have to go away again if she doesn't behave! She's still picking on the other pets there but they are being a bit more certain about their defence and Max is backing down. She's delighted with the change in litter box habits (Max was peeing on things and had really bad stinkies)! I recommended the litter I use and the food, and my friend is making those changes to keep Max happier.
Yeti and Sparks don't seem to have any lasting damage thank goodness.
They have come out from under the bed - are eating and using their litter boxes normally and have 'remarked' every surface in the house (twice!) so no harm done and lesson learned. I think we'll stick with the two for now.
Thanks to all of you for all the great support, wishes and advice - I guess it was just not meant to be. Silver lining - I can go visit her anytime I want!