Wow! Reading this is such a nice way to start the day! Two of the moderators here have also worked with disabled children and both have a houseful of disabled pets, look out!
What size are your pups, 15 lbs, or...?
Yes, I absolutely agree with what you said, expressing would be a huge help for her infections. In addition to expressing completely every 8 hrs (or more often if you like, every 6 hrs is good, too) I would put her in a diaper to keep her girl parts clean. It will help keep the dirt on the floors and pavements from contacting the female area, which can be a source of infection. I would get diapers with a hole for the tail big enough for stools to fall out, so she does not have fecal germs contacting her female area, either. Even if she continues to use a drag bag, I would still do the diaper.
You know, there are a lot of brands of drag bags, maybe she just doesn't like the kinds she has been put in before? This may be something you can discern by observation and intuition after you get her. If you want a list of some different manufacturers, let me know, I can get you some links. It's too bad she hates drag bags. You will have a better idea of what she needs after you get her. Possibly a diaper would be enough to protect her skin without the bag, but it is hard to tell in advance. Dogs drag in different positions, it's an individual thing. Some will drag on a hip, some will drag in a froggy legs position, and what position they drag will affect what area of skin is getting abrasions and what needs to be protected.
I think PT would be wonderful if you can do that with her. It would help her physically and also be fun for both of you.
Whatever I said above is just some ideas. You will figure things out when you actually get her, but I'm sure your idea on expressing is the way to go. You can also express the bowel. If you want links on expressing, don't hesitate to ask!