I don't see why it's tricky. If Americans in Afghanistan break Afghani laws then they are subject to Afghani laws are they not?

I see no difference between the ICC and Afghani courts in that respect. If Afghanistan wants it's domestic territory to be subject to ICC jurisdiction then that is the sovereign decision of the Afghani government is it not?

If Americans don't want to be subject to Afghani laws, which includes the ICC, there's a simple solution.

As for the UNSC referral that's not tricky either. All UN member states have agreed to abide by UNSC decisions and in America's case it has a veto over the UNSC anyway.

If a nation left the UN then the UNSC wouldn't be able to refer an issue within that countries land could it? The UN doesn't have global jurisdiction I thought?