Hi Nix, welcome to permies!
It's really helpful to include some basic location/climate info in your profile! For example, I have no idea if snow-load is a minor, major, or nonexistent consideration as regards these questions...
As you note, moisture and heat are the main complications, and the mold/decay/air quality issues that may result.
Keeping the greenhouse air separate from the household air seems very worthwhile to me. Good ventilation/shade is the obvious solution to excess heat. I'd like to try a
solar chimney on a greenhouse, but there are many options.
I would shy away from the dual A-frame you linked, unless you really need two units for something like two families... It's not very efficient to build separate buildings vs adding on to the existing structure, and the more wall/roof relative to interior volume you have, the more heat you'll leach out, for a given insulation value. A-frames are cute, simple, and good at shedding snow, but not very efficient on a materials/volume basis...
As far as adding a greenhouse to it, since the greenhouse would probably end up used as a hallway, this would be suboptimal in my opinion for isolating the airspaces. Plus, the two A-frames will be shading the greenhouse for parts of the day.
If you want solar gain from the greenhouse, and want to be really particular about keeping the moisture out, a heat exchanger of some sort could be used, or you could make the greenhouse-facing wall of something massive like stone, so that it would absorb greenhouse heat in the day and radiate it back in both directions at night.
I interned on a farm with an attached greenhouse about 7 ft wide, that ran the length of the house on the south side; maybe 40ft. It looked a lot like the one Ross built:
https://permies.com/t/48948/greenhouses/Dont-Build-Greenhouse-LOL
Notable difference, which I preferred, was that it had a riser wall so the glass didn't slope down so close to the ground; there was
enough height for at the front edge for plants ~2ft tall on a 3ft tall bench. Since it was attached to a 2-story house, it sloped up to below the upper story windows.
Like Ross's setup, it could be accessed from inside via a sliding glass door. It had exterior doors at each end for access and ventilation. It was a great setup. The cedar siding on the house was molding, but not badly, and it had been there for many years.
Either the greenhouse or the A-frame would probably need some modification to integrate the two, unless you build it along one of the non-sloped walls, which would limit length...