The comments re complexity, parts left out do not fail, etc. of course are applicable.
However: using retard only will save the engine - but without a few explosions for testing, only an expensive one (J&S Safeguard) will do so without reducing power so far that the run is wasted (i.e., result is you learned nothing, won nothing, broke nothing). Just backing off 20 degrees is probably safe, but how about 15? etc.
Retard alone will also raise chamber temperature (mix burning later in the power stroke = heat without mechanical energy) - which means it must retard more to offset the nitrous.
Water alone will reduce power but reduces chamber temp, so you'll need less retard for the same degree of safety.