1) ..................So which of the two sockets should I be using for my purpose?
They're identical so it won't make any difference which one you use.
2)............."Intel Optane Memory Ready" is this something I can benefit from?
Intel Optane is basically a hard-disk cache and is used to speed up the read performance of mechanical hard drives. There is unlikely to be any performance gain when the boot device is a (super fast) M.2 storage device.
3) Recommended appropriate M.2 SSD card for this purpose........M Key, type 2242/2260/2280 storage devices......
The Samsung 970 series SSDs are usually considered very good but you should do your own research to find the fastest model with good endurance that you can afford. Make sure that the M.2 device is compatible with your motherboard using the specs I've quoted above.