With an 18 GPM pump you will loose 7 PSI for every 100' of 1" pipe. A short piece of 1" is not going to be noticed. And that 7 PSI per 100' is at 18 GPM. With a CSV it will probably never see more than 5-10 GPM flow, which makes friction loss a moot point.
Remove and do not replace the above ground check valve unless you just want water hammer and all the problems that goes with?
#1 You would need the CSV1A.
#2 Don't know. Oldest versions of CSV's are 30 years old and still working. Sand will shorten the life somewhat though.
#3 Lol! You really don't even need a tank tee or either of those big tanks if you screw a 10 gallon tank into the 3/4 port on the CSV1A. With two tanks you really only need 1 tank tee for the pressure switch and other connections. The other tank can just be teed into the line with pipe.
#4 With Teflon tape there is very little connectivity anyway. But galvanized is the only thing I would not screw into brass, SS, or copper.
#5 Old copper is probably better than anything you can buy today.
#6 and finish #1 You can bypass as much water around the CSV as you want. I can also make any size bypass inside the CSV when needed. But I have spent 50+ years figuring out that 1 GPM is the perfect number. Why would I want to change it
now?