#1
C&D's previous test with the 20" tires measured an exceptionally short range that is inconsistent with what everyone here and on other forums is reporting. This new test with 22" is a only little low. Their huge reported discrepancy between the two is mainly due to their previous test being so out of whack.
Your range with the 22" should be 5-10% less than with the 21".
Your range with the 20" should be 10-15% less than with the 21".
The question is, less than what? For that the EPA range is a standard measure under known conditions. But because those known conditions don't necessarily reflect how you use your vehicle, that EPA range is useful mainly as a baseline, and needs to be adjusted on an individual basis for your driving style, for your normal speeds, for your typical terrain and weather conditions, etc.. Fortunately this personal adjustment is similar across all vehicles. So if you usually get better than EPA numbers on your current vehicles, you will probably get better than EPA numbers on your Rivian. Likewise, if you usually get worse mileage than advertised on your other cars, chances are you will get worse mileage on your Rivian.
The "real world tests" that everyone is so fond of are actually the worst way to gauge vehicle mileage/range. Because by definition "real world" means a non-standardized test environment that is only representative of itself - if how you use your vehicles differs (and it will differ - there are literally an infinite number of situations in which to test, and the chances that your personal usage matches that is, as a result, quite small) then there is no way to extrapolate what your mileage/range will be. And that's what people are looking for in these "real world" tests - they are only trying to figure out what their own result will be.
#2
If range is of primary importance to you, you will get the best range with the 21". The 21" are also adequate for your planned off-road usage. The 22" are relatively worse off-road. The 20" are relatively better off-road. I personally am getting 20" because I like the looks much better, I will be using them more off-road than you, and I would prefer to have the larger sidewalls for more safety margin. The difference in handling between the 20" and 21"/22" is totally irrelevant for the type of driving I do. I expect to get around 300 miles of range on the 20", like many others have reported, because I always get above the EPA rating the way I use my vehicles. That's more than I get with my current ICE adventure vehicle, so no range anxiety here.
Regardless, I think your choice of tires is a personal question that only you can answer. Which aspect is going to affect you most, range or performance? Both the 20" and the 22" are more specialized than the stock 21". If range is most important, get the 21" without a doubt. If off-road performance is more of a concern than range, get the 20". If on-road performance is more of a concern than range, get the 22".