Connie, you say that Tigger is drinking a fair bit of milk, this might be exacerbating the sores.  We had a cat that suddenly developed milk allergy and burst out in sores. Alfie didn't mind them being touched, and ignored them until they scabbed up and itched.  He wouldn't drink kitty-milk either.  Goat's milk is supposded to be OK, thought it was worth a shout to mention this.  Not saying it isn't toxicity, just wondering if there was an allergy as well.  Perhaps you should ask your vet about this.

We all rooting for Tigger and are really sorry that the sugery has been ruled out.  There isn't a lot that we can say except hugs and kisses all round.  Purrs from all our cats to Tigger!