Not really an answer to your immediate problem but it may help with Meesy picking on Tabitha, I also have 10 cats, (soon to be 11) and we have one called Polly Pocket who we thought was being picked on by the others, some days it was worse than others, we did even at one point think we may try and re-home Polly so she wouldn't get bullied, but I started to read about cats behavior and started to look out for things. Now it seems Polly isnt as innocent as she makes out, she is a lovely sweet cat when she is with humans and she certainly had me thinking she was having a hard time with the other cats, but Polly winds the others up quite a bit and I think they just had enough. Any cat that comes through the cat flap Polly gives a little bash round the head, she is always growling at the others under her breath (so to speak), Polly was demanding my attention and because I felt bad for her I was always pandering to her and letting her sit on my lap (all evening), I was always making sure she was getting enough food by feeding her seperately to the others and generally I was making a rod for my own back (and Pollys). The stuff I read said that I had to give all the cats equal attention, I had to feed them all at the same time (not if I had kittens) and I had to make time for all of the cats to sit with me and be petted. (common sense really but I was being too overprotective about Polly). After a few weeks of treating them all the same and not having just Polly on my lap at night and making her muck in with the rest at food time the bullying stopped, Polly is now happy and the other cats are happy.
The strange cat in the garden I am not sure what you can do about, I would try and see who it belonged to, have you seen the cat before ?, do you think it could be a stray ?