Some quick comments because I'm incredibly busy right now.
Narrow tires cut through fresh snow better to reach surfaces that provide traction. Wider tires tend to 'float' over the snow. Same goes for mud. If you lose traction, it means you need chains.
Narrow tires provide less rolling resistance, leading to an improvement in MPG. Wider tires have better load capacity.
The main (or only) concern with dual tire spacing is the possibility of the sidewalls coming in contact with each other. Tires spread out at the 'contact patch' with the 'road.' You need to make sure spacing is sufficient to avoid contact at that point. (Around 40 years ago, I was told you should be able to put your hand vertically between the tires, up to the heel of your palm.)
Fully inflated tires help prevent dual tire contact. Fully inflated tires also last longer, run cooler, and provide less rolling resistance. This is all why I prefer to keep my tires at the max inflation molded on the tire sidewalls.
Where you usually drive, in terms of weather and road conditions, vehicle weight, and how you drive all has a bearing on what's best for you. This is the basis for my tire selection ... and is very likely not appropriate for other people. (It'd take half a forum page to describe all the factors that went into my selection.)
Rim coining aligns the rim well enough on the hub. The alignment pins really didn't do much, which is why they were largely eliminated.
The coins on dual rims must 'nest.' The only problem I've ever seen is an idiot managing to mount the wheels with un-nested coining. (That person shouldn't have been allowed near a motor vehicle ... not even as a passenger, never mind a driver or maintainer.)
