I believe after 2006 or so, Nissan switched from the Green concentrate to the Blue 50/50 in all their vehicles.
Green was good but needed mixing and probably not in line with modern ingredients.
The 50/50 is very good and painless to use. All weather and seems to last longer. (I bleieve any 50/50 is good for -35c)
I would never mix the two for the reaons i suggest. It's ok, but I think you reduce the Blue's effectivity. Not a scientist or have the product ingredients, so it's just my belief.
I have used old (5 years) coolant, never noticed an issue. Noting failed prematurely. It is alcohol, so if sealed, should not evaporate too much. Thewn again, anything is possible.
There are devices you can buy that measure dilution levels, but I think that will just tell your mix ratios and what temprature they are good for. On the other hand, if your mix fails to meet the numbers, it may suggest your Coolant is failing or other problems
Ph can also be measured with Litmus paper. You'll have to research proper PH levels..