Snowshoe - When you've made the food changes in the past did you do it gradually over about a weeks time rather than just an abrupt change? Sudden food changes frequently cause vomiting and diarrhea.

The purina indoor formula is supposed to be less calories than the regular cat chow version. I'm not fond of the indoor version for several reasons particularly the dyes, however if it's truely the only food you can get the cat to eat without vomiting then I guess that's the food for Cocoa.

Cosaquin is a safe product with no side effects I've seen from the many cats at out hospital on it. Metacam was only approved for use in cats as a one time injection. The oral dosages are strictly off lable use. Many vets have used it in cats with various ranges of success but it can have side effects in cats as they can't process NSAID's the way dogs do. We don't use it orally in cats at work unless it's a last resort comfort medication, as in Dusty's case. Cats on Metacam should have regular lab work done to monitor liver/kidney functions.

I'd say that Cosaquin (with your vets approval) and a weight loss program is probably your safest and best method.

If you can't be home for a long part of the day, either separate the cats so Cocoa can't eat all the time, or have a friend/neighbor stop by a few times to feed them measured meals rather than free feeding.

Good luck.