(I'm sorry, I haven't read this whole thread yet, so apologies if I'm repeating things.)
One quick suggestion first: have you thought about getting a mattress protector?
I've learned a lot about stoma output lately, as I've just got out of weeks in hospital, where, as part of doctors' efforts to determine the cause of my weight loss,they were weighing all my stoma output, as well as recording everything I ate and drank, and weighing all my pee. I was eating and drinking a lot - I was having to manage a very high-calorie diet in an attempt to gain weight, and I was drinking quite a bit too, about two and a half litres a day - but my stoma still didn't need emptying more than a couple of times a day, almost always in the evening, which was the same as I was doing prior to my hospital stay, when I was drinking about the same, but eating a lot less. I've also found that the time I eat doesn't seem to alter the times my stoma is active. So I'm not sure about the link between amounts eaten and drunk and stoma output. Perhaps I'm an exception, but I would say don't allow yourself to become dehydrated through limiting fluid intake.
I do know that fibrous foods can in some people make output more watery. There are some foods which are supposed to help slow output down and thicken it - smooth peanut butter, marshmallows, jelly babies (something about the pectin in them!) and some other things. Once I've checked this thread more thoroughly to make sure I'm not just repeating things others have said, I'll dig out my diet guide and see what the other items supposed to thicken output are.
Another thing that happened when I was in hospital was that I did experience uncontrolable stoma output for the first time. It was caused by having NG feed, which is known to cause stomach upset, and for some reason my digestive system did not tolerate it well at all! So I had really watery output, and although I still didn't need to empty the bag all that often, for the first time I was getting leaks, especially at night.
The stoma nurses had a few suggestions - one was Derma-guard, which is an adhesive wipe to keep the bag stuck to the skin. This was actually really effective for me. Another was an extra big bag to use at night, which the output can drain into like a catheter. I was also prescribed a high dose of loperamide by the doctors, which did help slow things down. Luckily I was taken off the NG feed pretty quickly, as it was upsetting my stomach so badly I couldn't absorb all the calories it was supposed to be providing me with.
Final suggestion: before I had the ileostomy, I was taking Lomotil (co-phenotrope) in addition to loperamide to control diarrhoea. I found the Lomotil to be the stronger of the two, and as it has very few side effects, I could take a very high dose, and use the two together (along with codeine as well). But the strongest anti-diarrhoea medication I've ever had is Amitriptyline. I didn't take it as an anti-diarrhoea med, I originally took it to help me sleep, but it is also an anti-depressant and can help with stomach pain. I started taking it before I had the stoma, and it was the first thing to completely constipate me, and, after years of diarrhoea, I found myself having to take laxatives. If you want something to slow things down, it's definitely the strongest thing I've ever taken.
I really hope something I've said will help. But definitely keep asking your stoma nurses and checking out ostomy supply websites for other products to try. I know there were also many other adhesives available, as well as products which can be placed in the bag itself to absorb liquid. There should be something out there which can help you.