On the feeding times question, you may be able to figure it out better for your individual dog with the following tip. (It says 'dachsie' but would work for any breed.)
http://www.dodgerslist.com/literature/Expressing.htm wrote:
To know how long it takes for your dachsie to process food, put a few small pieces of raw carrot or kernels of frozen corn in with dinner. The veggie pieces don't digest — watch for the stool with veggies and you'll know your pup's digesting time.
Something I really noticed was the description of your dog's stool, kind of soft? Seems like a good idea to feed something low fat and with some fiber, but not all foods seem to work equally well with an individual dog's system. I have always had great success with Science Diet w/d dry kibble for disabled dogs, you get firm, low odor, non sticky stools and my dogs all seem to like it. The canned w/d, which seems like it should be much the same, does not give the same quality stools, too soft. At least that was the case last time I tried it, it has been a few years. But it doesn't have to be w/d, just whatever you can find that seems healthy and gives him stools you can deal with. Even though you picked a logical food, it sounds like it did not give the desired results and I think finding a brand that does work with your dog's system will make life much easier. As far as I know, if you don't like the w/d you can return it for a refund, or some vets will give you a baggie of food to test on your dog instead of buying a whole bag.
If you really can't find anything that works for him, you might want to ask the vet, because dogs with mobility issues should really have the same quality of stools they always had, so you might want to be sure nothing else is going on. I can't imagine what it would be, but sometimes a senior wellness checkup can give you peace of mind.
I agree on expressing stools, that's big help, let's you take control of the situation to some degree, however you may still find some doodles between expresses. With my old dog, even if he doodled in his bed, and even if it rolled under his hip and he laid on it, it was not a mess, he did not need a bath, and the house did not stink. So I was really happy with it (meaning the w/d).
Also, I am attaching a photo of a smaller dog, but it gives you an idea of how to keep diapers on. There are a lot of different kinds of suspenders you can try. For example, if you put him in a nylon chest harness and attach the diaper to the harness by means of bedsheet fasteners or something similar, it will help keep it from falling down. Another possibility is the brand of diaper. Diapers do not all fit the same. I can't say offhand which adult diapers come up higher in the waist, but it might be worth trying a different brand to see if it is any better?
There are some companies that sell diapers made to stay on. Here is one that has been around for a long time.
www.joybies.com
One other idea, which I have just been doing recently, is to express the bowel twice. For example if you feed him and express his bowel, then wait a few hours and do it again right at bedtime, you will probably at least minimize the overnight situation. I have found with my one dog, even after I express everything that I can, if I go back and do it again a couple of hours later, I get about the same amount the second time. It helps. Even then we wake up with some doodles in the bed, but I know it's considerably less than it would have been, and that is when you are glad for "quality stools".
. Thanks again and it just feels comforting to have a place to discuss these issues and know others understand. He is my first old man that had experienced this so this is a whole new world for me to understand. I'm definitely going to try helping him express his bowels before I leave for work and before bedtime. 
How is expressing poop going?