First of all, welcome to pets.ca! Sorry to hear about your cat's diarrhea. I think a checkup with the vet would be a good idea.
Do you home test your cat's blood glucose levels at all? If so, what kind of numbers have you been getting? The reason I ask is because Lantus, even though it's long-lasting for people, is rarely a once-a-day insulin for cats. They metabolize insulin much faster than dogs or people so bid dosing provides the best coverage.
Also, about that Hill's M/D: not the best quality ingredients, and to be honest, diabetics shouldn't be eating any dry if possible. You don't need prescription food, any low-carb canned (or raw!) will do. Having said that, I wouldn't make any diet changes unless you are testing bg's at home because removing the dry could decrease insulin needs, resulting in a hypo if adjustments aren't made.
In the meantime, you could try some slippery elm bark for the diarrhea and to sooth the intestinal tract. There's info here on that: http://www.littlebigcat.com/?action=...em=slipperyelm
I hope your kitty gets better soon. Give us an update on her condition.