It is probably more the production of the amalgam than the nitrate itself. It has less to do with the aluminium foil itself, as long as the right amount is present. You can tell if your nitrate is effective if the aluminum slowly dissolves and the water turns dark gray and bubbles rise.
You have to wait for the right moment when you add P2NP, when enough amalgam has formed, but before this reaction reaches its peak, I would say 10-15 minutes after the addition, but it always depends on how much nitrate you have added, how much you stir or shake and how your aluminum looks like.
You said everything would have evaporated, the reaction is strongly exothermic you have to cool the flask to get that under control, so there you were on the right path.
The nitrate liquid is an acidic solution and if you have prepared it according to the instructions, it can be stored in the refrigerator for several weeks/month. And yes, the nitric acid must be hot, at least 60°C so that the correct nitrate is formed with the formation of brown toxic nitrogen dioxide.
If you are from Europe, take a closer look in the eastern part, there you can get mercury and sometimes the right nitrates.
You can scale the quantities as you like, but keep in mind that if you don't succeed in synthesis with the small quantities, you won't succeed with the larger quantities either, and then there is the danger that the reaction, which increases expotentially with the quantity, will blow up in your face, and we all don't want that, do we?