Did you at least try doing a simple google search on any of these questions.......
1.sprit pcb is like revision 1 of the facew/B.face. in short sprit < bface
2.the sprit looks like it can support under side led's but you will need to solder an ws2812b led strip. You will need to solder it to the power and data holes near the usb. A ws2812b led strip can be bought on ebay.
3.idk
4.mechanicalkeyboards.com, originative.com, ebay.com