Orthodontics while changing dental insurance and FSA question

Hello. It’s my yearly benefit election period at work. I currently have a Delta Dental plan that pays $1000 lifetime max for orthodontics. My son got braces earlier this year. After the insurance paid, I still had a balance of about $5400. I had to pay a down payment of $500 and I pay $221 a month for the next couple years.

This year, we have a new BCBS plan being offered that has better benefits. So I’m selecting the BCBS plan. It covers everything better except still has a $1000 lifetime max for orthodontics.

Question 1: Will switching insurance half way through screw up my current payment plan? or could it actually be beneficial, as in would BCBS pay another $1000 on top of what Delta Dental has already paid.

Question 2: I can contribute up to $2850 to an FSA. With how much I’m paying each month for his braces, I’m pretty much guaranteed that I could max out my FSA contribution and use it all up before the end of the year. Is there any reason not to do this? I’m a bit hesitant to contribute that much to the FSA and then have them come back and say something dumb like since the braces were put on this year they’re not eligible or something. I called my FSA customer service and asked and the girl I talked to said that it would be eligible, but she didn’t sound very confident. Then she said she would double check and came back and again said that it should be eligible and if it gets denied to call them back and ask why it denied… Which is not very reassuring.