To be clear upfront - I am most certainly NOT a water chemistry expert. However I've been reading up on it a lot. (and I must say I have really been enjoying Martin and AJ's ongoing good-natured bickering - y'all are like an old married couple, but with Ph.D's
1. I've never used the 5.2 stabilizer, but most everything I've read says it doesn't do what it's supposed to do. I have no idea why -- or if that's even the case. But if you so some searching you'll find more info.
2. There was another suggestion at the beginning of the thread that I'd like to highlight. One way to get rid of your pH concerns for the stout would be to leave the roast grains out of your mash, and blend them in right at the end (or even steep them and add the steepage (?) separately). That would make the grist more like a pale ale malt bill. I haven't done this myself because I, uh, only heard about it a couple days ago. But quite a few folks do it and I can't see anything that would cause a problem.
Just a few things to consider by way of an over-read noob...