The FHR program only requires hotels to give a one category upgrade. This can mean that you won't be upgraded if the only available rooms are two or more categories higher than what you booked. Hotels can also specify the maximum category upgrade they will give, so that for instance a junior suite won't be upgraded to a standard suite even if this is the property's next category and even if standard suites are available for the length of your reservation.

The 4 pm checkout time is guaranteed. IME AmEx takes the FHR program requirements seriously. Whenever I've had a problem, a quick call to PTS/CTS always resulted in the problem being fixed and the hotel dinged for their behavior.

Also IME different hotels have different policies regarding the $100 credit, so be sure to ask some employee who knows. In this case, I would also get the employee's name or try to get clarification by email. I know of hotels where it was a $100 spa credit, not applicable to F&B.