On the baby diapers, hopefully one of the members here with pugs will know exactly what is best. There is a diaper size chart here that may help. If you find a size and brand that works for your dog, please let me know and I will add the information to the size chart to help the next person! http://www.handicappedpets.com/mediawik ... male_wraps
I do not use disposables with my dog, I use a denim diaper with a pad inside. The diaper fastens at the waist with velcro. All I have to do is stick a fresh pad in it as needed. Since my dog is small, I use a moderate incontinence pad for humans and cut it in half. When it is time to change her pad, I pick her up, pull the diaper off with my free hand, put her in her playpen for a minute, stick a fresh pad in the diaper, and put it back on her. It is very simple. I find it much easier than when I used to use disposable diapers for my bunny.
I cut the pad in half and put the cut end in the front of the crotch and the sealed end of the pad by the tail. This way the stuffing of the cut diaper doesn't come out. (My dog has 3 legs so I angle the pad, that's why it looks crooked.) I put the pad so it actually overlaps into the tail hole a little to prevent leakage.
I am trying to think how you can test the diaper on your dog. I think if you take a human thin menstrual pad and cut it in half and put it in your purse and take it to PetsMart, probably you could open the diaper package, place the pad in the diaper to keep it clean, and try it on her for waist size. Those denim diapers are kind of expensive so it isn't something you necessarily want to buy to try, it's better if you can try it without having to buy it.
The way I put it on her is I lay it flat on the kitchen counter (left side of sink is sanitary cooking side, right side of sink is all else) and stick the pad in. Then I sit her little bottom on the pad. This way I can work at waist level to hold her still while I fasten the first velcro tab with one hand. When that is done I can use 2 hands to fasten the other tab snugly. I am sure it is easier if your dog can stand up, but you get good at wrangling on that diaper one-handed with just a little practice.
I've never used bethanecol, I hope someone else will reply on that.