Hmmmm.... Also, if your kitty is still getting constipated on that much lactulose, then it seems like the lactulose isn't really doing what it's supposed to. My cat gets between 4-5mls per day, and the vet I've started dealing with lately said that much lactulose should be doing the trick. I'm going to start the canned pumpkin thing and see if she likes it. Is that what you started adding, or are you using a different fiber source? My understanding is that canned pumpkin can go both ways as far as helping with constipation and diarrhea. Plus, cats can be constipated and still have diarrhea, because soft stool can push out PAST the hard stool that's built up. I learned that the hard way when I was just about to leave for work one morning and stepped in a huge pile of poo.
That was one of the least fun things that I've ever had to clean up.
I brought her to the vet, and he could feel solid poo in there when he did a physical exam, even though she'd just had diarrhea the night before. He thought she might have had poo with hair in it stuck in her colon or intestines or something, so I ended up giving her laxatone to aid with that. I've heard that it shouldn't be used long term though, so I give it to her every now and then.
Has your cat officially been diagnosed with megacolon? Like, does he have an enlarged colon? With megacolon, the colon can't contract properly, making it difficult for a cat to poo. They have motility drugs for that reason, but that's only if your cat has been diagnosed with definite megacolon or some other condition related to decreased motility.
You mentioned something to me about IBD, so I wonder if Jordan might have that. It's hard to say. I've also been told that relatively large doses of lactulose (the vet was referring to 6ml per day) shouldn't cause nausea or vomiting because lactulose does it's job in the lower GI tract and has little effect on the upper GI tract. I'm not sure if I agree with that, because my cat has puked up nothing but lactulose on several occasions. Then again, my cat could have an ulcer somewhere in her upper GI tract, or she may be producing too much stomach acid for some reason.
As far as the vet vs. dry debate, I'm really not convinced that dry food helps their teeth. I had a cat growing up that ate nothing but Friskies and Tender Vittles, and my parents said he'd never had a dental in his life (and honestly, the vet we went to at the time was awesome). He just had good teeth. Then my Bailey was fed nothing but dry food for the first 8 years of her life, and her teeth were pretty bad when I first got her 4 years ago. I think it really does depend on the cat, genetics, and the quality of food (regardless of wet or dry). I give Bailey about 1 can a day, split between 3 meals, and I leave dry food out all day (1/2 cup per day, but I put down 1/4 cup in the morning and 1/4 cup in the evening to make sure she doesn't gorge herself with it in a short period of time.) I think I have noticed a connection between eating dry food and her constipation. Like, if she goes through the night and eats a ton of dry food, she'll have a tendency to get constipated. I don't know, it's a balancing act, I guess.
One of the reasons I give her wet food at pretty regular intervals 3x per day is to make sure she's regularly getting accurate moisture, which she'll get from the wet food. Maybe you could try splitting up her wet food into 2 meals, and then feed her dry during the day/night after she's eaten her canned food? Then again, I know you have multiple cats, which definitely complicates it.
People who say that cats are low-maintenance animals would certainly be proven wrong if they had out cats!