That's a lot of questions for a simple subject line, Brett!
Just curious what magical drug combo your wife was advocating that would cure all your dog's ills....?
To get further with your dog's joints, you may need to pair with a Blood Mover like Xiao Huo Luo Dan. I'd give that a try
Regarding corn starch in the probiotics, if it's not a major amount, then it's probably okay. Probiotics do best with a fermentable carb food source, anyway, and your dog is already on kibble, so the added starch exposure may not be significant enough to worry about.
Regarding probiotics, I'm not enough of an expert to have a favourite brand. My own bias is to use species that are known to produce compounds that favourably modulate the immune system, like Lactobacillus and Bacteroides. Lactobacillus is apparently a component of the VSL products, so you may not need to switch if you like them.
Probiotics do better when given with an immune stimulant, but you have that base covered with the BZYQT and the XCHT.
So, bottom line. The probiotics may not be the missing link here. I'd wonder if a Blood mover might solve a lot of problems for you. I'm guessing your wife is leaning towards NSAIDs? If so, then XHLD is a better choice, since it has a lot of the same (albeit milder) effects, but doesn't hinder joint circulation, inflammation resolution and healing
Hope that helps
Steve