I read a statement @Hans-Werner made (don't remember the post because I have been reading many) but he stated you can not use plus NP to raise measurable N and P because the ingredients are not measurable using hobby grade test kits yet I keep reading people talking about testing when dosing plus NP so I'm a little confused.
This is especially true for the majority of nitrogen compounds in Plus-NP since a good proportion of N in Plus-NP are organic nitrogen compounds.
Phosphate will be found after a few days. The dosed phosphates will not be found immediately in full concentration. After enzyme enhanced hydrolysis it will be found also by test kits. Corals, algae and bacteria under low phosphate conditions excrete the enzyms that will enhance this polyphosphate hydrolysis for takeup. This will occur in a few hours.
In a range of days the dosed phosphate will occur in the normal "phosphate metabolism" of the aquarium and will be just as easy or diffucult to find as any other phospate. So if you continue with regular Plus-NP dosing the full phosphate concentrations may be found with a maximum delay of 1 or 2 days, but this should be no problem at all.
If I understood well, dosing NP Plus the PO4 test have to detect an increasing of the phospate.
Do you confirm?
Yes, this is correct, as you can read above. Our instructions are generally adjusted to a possible delay in finding the full phosphate concentration.
Nitrate and phosphate are available to corals. Might other forms be "more" available? maybe.
Regarding nitrate please see
here and
here. Maybe the best source for nutrient effects on corals still is the
dissertation of Shantz at the Florida International University.
I did my first trials with reduced and organic nitrogen compounds on one side and nitrate on the other side more than 25 years ago and the results were in perfect agreement with the images and results given by Wiedenmann, D'Angelo et al. in the first two links. The trials described there are aquarium trials.
Nitrate may have the negative effects described in the links, especially under low nutrient conditions. Nitrate is easily throwing nutrients out of balance when phosphate concentrations are low. This is why I don't advocate dosing nitrate as the only nitrogen source.
P. S: Just after I had written these lines I had an e-mail about a
brand new article on exactly this subject matter. This was really "just in time".

Phosphatase enzymes are the enzymes that hydrolyse both, organic phosphates and polyphosphates dosed with Plus-NP.
I am extremely surprised about the 100 % agreement with the lines I have written just minutes before. I have contact to Christine Ferrier-Pagès from time to time, but I don't know how much our short discussions on nutrients influence the experiments of her lab.