1: Fits right in. However, if you already have a complete GSR block there really is no point in swapping to LS components. Despite the enormous boners many people have over that 40-some CCs of displacement, #1 it's not worth it and #2 the GSR's OEM components are lighter and better balanced.

IMHO "being different" isn't worth a complete teardown just to throw in a different crank. Plus, the VTEC engines have a larger aftermarket.

2: CTR pistons with GSR rotating assembly yields somewhere around 12.3:1 - 12.5:1 compression ratio (depending on who you ask). I would highly recommend not going over that CR on pump gas, as you will quickly find yourself running into detonation issues without excellent tuning. Besides, if you want truly obnoxious compression ratios, custom pistons is a far better choice. Milling is permanent.

CTR pistons do have large reliefs, but their dome is higher.