I'm in the same boat, and I would love to know what you end up finding out. Being just 10 additional nights from achieving Platinum via 50 nights vs. the challenge, and thereby being able to select an Annual Choice Benefit, I am thinking about ignoring the challenge and getting those nights whenever.
did you read the T & C ? That's the only thing that matters versus what you have heard....
It's been a while since I looked into challenges (Platinum or Gold), but I do recall only paid stays count.
Marriott has always treated Cash + Points stays as an award stay. What they do is assume that the cash portion covers 'buying' the additional points you need to complete the stay. For example, on a category 4 hotel, the normal redemption rate is 25,000 points. When you use Cash + Points, it's 12,000 points + $105. Marriott is basically letting you buy 13,000 for the discounted price of $105 (normally that would cost $162.50).
Cash + Points stays have never counted as paid stays for the purposes of promotions, so I feel confident saying the same is true for challenges.
In this case, what the rep told you is correct.
Retrieving data ...