My experiences having done this route frequently:

The plane will depart ORD from the low C-gates. This is where they have the E170's with F class.

You will pre-clear US Customs at YUL before going through security. The lines may get long, depending upon when you depart. The forms are available at check-in. Some agents check to see if you have indeed filled them out before proceeding.

Security is not TSA, but the same rules are basically followed (including the disgusting shoe removal). If you buy duty-free, it MUST be put in your checked bag if it is a liquid more than 3.4 oz.