OP, I fully understand your thinking and preference for a complete OEM hard cab. I have a hard roof, soft doors, and hard rear panel on a P1000-3. Also have an inferno heater. I have no knowledge or experience with either a complete hard or soft cab enclosure but I'll raise several points to consider.
1. Air intrusion through the shift lever slots. A significant volume of cold air comes in that way.
2. My Honda OEM two piece windshield does not seal completely against my Honda OEM hard roof. The roof has molded raised portions - probably to increase strength. Cold air intrudes between the top windshield frame/ROP and the raised portions of the roof.
3. An enclosed cab and cab heater will provide a great improvement in comfort in any cold weather. But even in cold climates there are warm summer days. You'll want cooling air flow through the cab. Opening the upper portion of a two-piece windshield and the rear window slider of the hard rear, and the hard door roll-up windows will help but will it be enough?
4. My state of WV allows ATV/UTVs to be registered and driven on public roads and highways if they have street legal equipment. Three (3) rear view mirrors are required. Inside and two outside.
Honda P1k soft doors are simply not intended to work with outside rear view mirrors as designed and built. I've purchased a pair of every cheap, plastic ChiComm piece of crap outside mirrors I've found on the web. None of them are satisfactory. They don't reach far enough out from the cab on the right/passenger side to be seen completely from the driver seat and the mirrors do not have sufficient range of adjustment to see cars behind the buggy unless their within 10' because of the lean angle of the windshield posts.
Honda OEM accessory mirrors mount to hard doors and apparently work well. They can't be installed on soft doors.
The hard truth is the current generation of Honda Pioneers, with many features I I like and appreciate, are simply not designed and built to provide weather sealed cabs. Sidebar: I would not own another brand with rubber band drive. Imo they're junk.
Many "band-aid" fixes are available but nothing I know of will completely seal Pioneer cabs against cold weather. Is that critical? Not for me. Winters in southern WV are mild and the engine generates enough heat after five miles of driving to provide adequate comfort. The aftermarket Inferno cab heater with engine heat is all I need. A wife and kids might prefer more on a ND winter day but with you, your wife, and kids in the buggy along with the two heat sources I mentioned I think you'll be fine.
Personally, my choice is hard roof, hard rear panel, oem two-piece windshield, and soft doors. Your choices may differ and will hopefully please you.
I'd like to hear your opinions about your eventual choice.