I have the 44Hv2 and the M37C. I do not find the 44H harder to play than the M37C; just the opposite.
It’s nice that you have three Hammonds to be able to experiment with. If indeed you put most of the 44H parts on the 44HP and obtained the tone of the 44H but the ease of the 44HP, then about the only place to left to look would be in the hidden chambers below the reeds and on to the exhaust ports. And the only way to check for differences there would be cut open the instruments! Ok, I suppose you could remove the reed plates and get a partial look.
You can hear me playing a 44Hv2 at https://youtu.be/jwUalR6XWjs. I’m not sure if that will give any insights.
Air distribution is a problem in most melodicas. It seems that the low notes are always stealing the air—the wider the spacing, the more the steal. Oscar has a way of balancing the notes on his custom designs. If you really want to hear balanced notes, though, get a Y-connector and hook up two separate melodicas. Play the low notes on one and the high notes on the other and you can hear truly balanced notes.
I have experience with only two models, though, so there may be commercial instruments that do as well as my gold standard (the two melodicas).