I have booked several of these types of "mixed message" reservations at Marriott, such as the one described in detail by the OP.

Only one time I needed to cancel it, and when I did it on-line, it went through the same as any other cancellation that would occur if within the time period stated. No charge before or after. No special calls to property or to Marriott.

Remember that the property is the party drawing up the reservation terms and conditions, and in case of ambiguity or conflict (in the absence of language stating otherwise), the benefit of the doubt goes to the party (the guest) who did not write the T&C. So the most favorable condition of the contract must be honored.

This is not the same as an obvious mistake rate (e.g., $1.14 per night rather than $114) that a hotel might not choose to honor and have a leg to stand on because what appeared was unreasonable by any measure.

One thing to check (before getting a telephone rep in the mix who can make this attractive aspect of the reservation be removed because you're pointing it out), is if your card in fact is charged in advance. Often with these "pre-pay non-refundable within one day but you can cancel without penalty up to one day before check-in rates" there is not any pre-payment taken. When you show up for your reservation, the charges begin.