Agree that you will not be able to get a US call so you had better get your ticket at home which gives 2M privileges.
As far as getting caught & being locked up for pirating, not likely. The FCC don't even go after their own people who have been doing all kinds of illegal stuff on the air for years & years after many many complaints & notifications. They won't be assigning a whole division to track you down for exchanging a few pleasantries on 146.520, or any repeater if you don't have a reciprocal.
Here in Bermuda we get many visiting US hams who bring their 2M radios. Do they all get reciprocals? I seriously doubt it. Do I check? No. Do I care? No. As long as they are using a legit sounding call sign I am happy to have them on the airwaves.
Now, if somebody suspects that you do not have a license from your home country then that might be another matter.
On the antenna question, that GRP roof will be a problem. I once had a glass mount on my car. It had its limitations but it worked.
