Answer 1: You should do pH buffering (adjusting) last. Put all the nutes in your water then test it...that'll give you a starting point for what you'll want to do.
Answer 2: Yes you can....but if CaMg is part of the recipe, you'll want to put that in first and let it sit for a minute or two, then add the rest of the nutes.
The pH of your feed is THE PRIMARY way for you to make adjustments of your soil pH So as such, you need to know what your soil pH is in both the wet
state and the dry state...this is known as your "swing" or your "range". Soil will usually be higher pH when dry and lower pH when wet. You want these
numbers to coincide with the low and high of your desired pH range. In soil, that's roughly 6.1 to 6.7. Once you know the pH of the soil in these states
then you have the ability to tailor your feed solution to make adjustments and dial in that range.
Example:
My Soil pH is a little high right now...6.7 wet. So I feed 5.8-6.0 pH'd solution to bring that number down. It only brings it down significantly for a little
while then the soil restabilizes so the permanent effect is much smaller than the initial effect. But by doing so, I brought the pH down enough for the
soil to be in better range for the uptake of Iron (in the form of molasses) to handle what may be an Iron deficiency.
Hope that helps
