I do not believe that your assertions are correct, either in how and why Prime works, or the potential for interference. It does not primarily detoxify ammonia simply by converting NH3 to NH4+. Any acid will do that, including perhaps Prime itself, but it would take a huge pH drop to be effective that way. A drop of 0.3 pH units will only drop NH3 by a factor of 2.
Measuring ammonia after Prime is problematic.
Nessler types of ammonia kits (yellow to orange colors) used with Prime can lead to brown colors that are off scale and are not necessarily an indication of free ammonia.
Salicylate ammonia test kits (yellow to green) can break apart the ammonia/Prime complex and make it look like ammonia is present when it is not (according to Seachem).
This is what Seachem says (among other, possible technically incorrect things

:
http://www.seachem.com/support/foru...512-prime-and-false-positive-ammonia-readings
""Under the conditions of a salicylate kit the ammonia-Prime complex will be broken down eventually giving a false reading of ammonia (same as with other products like Prime®), so the key with a salicylate kit is to take the reading right away. However, the best solution ;-) is to use our MultiTest: Ammonia™ kit."