1) Yes, we crossed over this threshold last year. As our incomes increased, the math switched to traditional. Probably the math indicated traditional for a while before that, but it is good to have my backdoor Roth pipeline in place as you mentioned.
2) Yes, if the math indicates it, simply put your $5,500 in the traditional IRA account this year instead of the Roth. You'll save your marginal 2017 tax rate by doing so.
Bonus points if you ask your broker to recharacterize last year's Roth contribution and move it to your traditional IRA. I did this in 2017. There's a form you fill out and a special tax document you receive. Do it before April 15.
3) Yes, you can contribute up to $5,500 into any combination of your Roth or traditional IRAs. E.g. $5500 Roth and $0 Traditional, $2750 Roth and $2750 traditional, or $0 Roth and $5500 traditional.