Interesting question. I don't have the answer to your theory, but I know the actual time frame is 3 months not 6.
I did the same to get the BMWFS rebate and then paid it off in 3 months. Also BMWFS will not come back on you, but the dealership, so depending on your ethics you can essentially do what ever the hell you want. I'm not sure on the minimum balance needed to be maintained, you may ask your SA, but sounds like they already gave you the wrong info on the term (assuming it hasn't changed in 12 months).
Also for the "new car" question, my credit union considers anything 2007+ as "new" not just a 0 mile car sitting on a lot, I would make a call to your bank.
Herbert - '12 X5 35d / Sapphire Black / Black Perf Nevada / Aluminum trim / ZPP / ZPS / ZCW / ZAV / Apps / 3rd row
Betty - '04 CTS-V / Black Raven / Black Leather & Suede / 6 speed LS6 combo 400 hp & 400 tq
Sterling - '06 Silverado HD / White / Grey Leather / LBZ Duramax - Allison 6 speed combo / Crew Cab Short Bed LT3