In my experience the small ones are powerful enough. After filling the tyre, and the pressure is established, I use a gauge to drop it to the desired pressure.No. I suppose if your tire is already full of sealant CO2 cartridges would be better. Just make sure to get the big ones.
I envisage riding these events with 2 x tubeless full of sealant and maybe the inserts that @OreoCookie pointed out. Then maybe two spare tubeless tyres incase the tyres rip, and then a handful of tubes for anything after that. Plus a load of cartridges and the gauge...
Definitely sounding more and more like a survival adventure ride than a race!