They are going to be on my call list this week, I feel 5500 rpm is the rev limiter because it doesn't fluctuate any even if you adjust the trim either direction, but we have had the boat it the water once for about a 30 minute trip and this is my first outboard and first pontoon, the engine is a demo off of a saltwater twin engine boat, I feel certain it had that prop on it for that use.
Monohulls with less drag can go down in diameter and up in pitch, so that 14.25 x 18 makes sense if coming from another boat. For our application (again, my opinion only) with more weight and drag a little more diameter is necessary to give the prop that "bite" and reduce slip. Do you have a 3 or 4 blade? But that RPM issue is made even more curious by the fact you can't get more by trimming out. Maybe the tach is bad? Was this motor adjusted in some way to synch to the other but now needs to be decoupled? I'm really just making stuff up at this point, so take it with a grain of salt. Working with the dealer and OEM at this point is the right course of action - demand WOT RPMs and let them figure it out for you.