Pumpkin can be mixed in with Shoes' wet foods. They tend to like it. I think the amount is usually about 1 tsp to start off with. Yogurt...hmmm...not so sure about that one. Never tried it. I usually use FortiFlora (probiotic in powder form), sprinkled over her food. Hopefully, someone with experience will chime in here.

As for the mucousy, loose/soft stools, it's a form of "colitis". Try not to switch foods around too much. Cats and dogs can get digestive upsets just from changing too much...even if it's the same type of foods (i.e.;, wet to wet or dry to dry). But, I also know that you're doing all that you can to get Shoes to eat so, it's 6 of 1 and half a dozen of the other, huh?
Usually, what's helped Morrigan in the past with this (trying to get her over to canned foods, which never worked as this kept on happening

) was Flagyl/Metrondiazole (sp?) then, getting her back onto eating a food that didn't upset her digestive tract again and letting it all smooth out with some FortiFlora (probiotic). But, if you can do that with Pumpkin....great!
BTW...not sure about this but, did you happen to notice on the yogurt if it was "lactose free"??? Even though it has the bacterial cultures/probiotics in it, the lactose *might* make it worse. Maybe, try just the pumpkin for today and tomorrow, when stores are open again, try either a Lactobacillus-Acidophilus probiotic from the health food store or any good pharmacy? Or, perhaps, picking up some FortiFlora from a vet's office?
ETA: Merrick can be pretty rich food so, if Shoes isn't used to it...it can cause this no matter what flavour you have.
Hopefully, someone with more experience with this will chime in. Shame that it's a holiday weekend for everyone as people are busy right now with dinners but, hopefully...someone will be by soon.