Just two days ago, I was able to help a friend retroactively sign up for the Platinum challenge; even though she has already signed up to start her platinum challenge on 2/16. By the way, she earned her gold on 2/15 through a challenge started on 1/1.

It took about 20 minutes being on hold for several times, and the rep told us that it can be done, they are just have a bit of hard time getting the computer to do it.

Everything eventually got sorted out, and my friend now has a platinum challege starting 2/1, and the flights she took since 2/1 was included in the Q-point total that the rep tallied for us.

No need to lie or imply, just ask for the challenge to start earlier that when first requested. And YES, in our experience, the Q-points do count retroactively.