Tires

Probably not. Replacing two tires with one has two potential advantages. One wide base tire typically weighs less than two conventional tires so, if you max out your load before you fill your trailer (not likely in your case), wide base tires can help. The other potential advantage of wide base tires over conventional dual tires is fuel economy. Unless you are hauling horses from coast to coast, the fuel cost saving would be difficult to measure. Also, there are fuel efficient conventional tires that can offer similar fuel cost savings without the cost of changing tire sizes.

I assume you mean a vehicle that has not moved for a very, very long time. Just sitting overnight can often cause the tires to have a slight flatspot on each tire that will tend to go away within about 15 minutes of highway travel. Vehicles that have been sitting for months or years can cause a more severe flatspot that just won't go away. I can't image anything you could do to get rid of these kind of flatspots.

A tire with a higher load range is designed to be run at a higher inflation pressure. So, it could be heavier and have a higher rolling resistance value because of the extra weight and is constructed just a little different. But, even if there is a difference, in my opinion, you would never be able to measure the differance even if you track fuel economy very closely.

I suppose it is a matter of opinion but, I think you are correct. A metal valve cap seals the valve and dirt, water and other undesirable elements from getting into it.

All steel tires (tires with a steel body ply and steel belts) are considered "commercial tires" at least in Goodyear terminology. This means that the are intended for use on "working trucks" on a daily basis. Tires with fabric body plys and steel belts are called "consumer tires". Occasionally, these tire might be used in some sort of working environment but, for the most part, they are on vehicles that provide transportation to a driver and a passenger or two.