Let me try to get my mind around this.
Shop guy says your dirt should be 6.5
Your runoff is 5.5
Your water is 7-8
Yes, there is buffering capacity in soils. HOWEVER that doesn't add up!
It would make sense if your SOIL was 5.5 and the RUNOFF was 6.5 with neutral water.
Cheapo meters are unreliable, but if you can get a cup of dirt, and a bottle of distilled water, and mix them and let them stand for a while, you can get a better idea of where the soil starts.
The only explanation for your really low runoff that I can come up with offhand is that the runoff includes soluble salts, which may be acidic.

Again, why I avoid peat moss and lean towards leaf humus... how is that compost project coming there big guy?

The health of your plants is going to be the final judge. After 2 years of growing, low soil pH is one of the most apparent problems to me now, having lost an entire crop to it once... that just gets burned into your mind! So I'm super sensitive to the symptoms... obvious Mg lockout, blotchy weird colored low leaves, strange colors in other places... purples and yellows where they shouldn't be...

I think you are going about this right by pHing the water first. It's going to save you some headaches down the road.

Anyway yeah.. sorry I've been so out of touch in the last couple weeks! I'll get an email off to ya soon, promise!