Yes on both counts - the only difference between the NPAs is volume - the 15ml is the more cost effective. Also the “half strength” A1 from LS is actually the same strength but in a 5ml quantity.
Aso remember that Bruce at LS has stated that A1 is already in NPA and other Lacroy products but he has never given a hint at proportion/amount.
Yes, diluting both of them is best - The NPA dropper is in no way accurate and you do risk rampant OD/smelly problems by using it straight, it is also pretty poor diffusion from a drop application. Mixing with a moderate strength cologne is the best way IME. Lighter colognes (eg CK Eternity) will only provide effective cover for the lowest dilutions.
As to what to use - This is where you need to do some testing yourself - “Standard” dilution NPA is 1ml NPA plus 5ml cologne but at my age I’ve used all sorts of doses and had results with 1+4, 1.5+3.5 and 1.75+3.25 in differing strength colognes and with different numbers of sprays. These days I mainly use 1+4 in moderate cologne and 1.5+3.5 in a pretty potent one. There are good threads various tactics for using NPA here already.
As a younger guy, you may want to go lower still but that’s up to you.
Much of the above holds true for A1 and initially anyway, you might want to mix that in a separate togo until you find the dose/proportion that works for you.
If you have not already read K-Trains excellent A1 guide, I suggest you do so now - it contains all you need to know. 
Also the A1 Mix Repository:
Measuring pipettes are usually sold at pharmacies and if you don’t fancy paying a dollar a pop from AD, you can get a box of a hundred for 5-6 dollars on Ebay. Similarly, if you don’t have any togos, Sephora atomisers are excellent and can be got at most good perfume shops - Their spray volume is known and a few minutes on search here will turn that up.