You aren't the first person who has posted that a CM told them that dates were sold out, only to find out that they could still be purchased online. I think it is a real shame that CM's are giving out bad information.
I would go to the Disney website to order the tickets online, and then email customer relations, conveying exactly what you posted here. There is NO REASON for a rude CM, that should be reported, in addition to the fact that CM's are telling you that the party is sold out when clearly you can still buy tickets online for the same night.
I'd be really frustrated. Email them (and hopefully you'll get a satisfactory response) to hopefully put this behind you before your trip.
Have fun! (And enjoy
MNSSHP - get those tickets online directly from Disney, and enjoy it!!! Our first MNSSHP is 9/15 - I'm getting so excited that I just can't stand it.

LOL)