If my ETS balance is 42.5 days, and I get out in February 2018, am I allotted all 42 days for transition/terminal leave?

Well you have your answers, bottom line up to Commander, I myself put in for over 60 days when I retired. I had 60 on the books and earned leave while on leave, It was approved.

Leave time is yours and your alone.
The commander will have to approve anything over 60 days, by regulation if you are not flagged you can take leave,
terminal leave is the same way , it’s your call on the way you take it, use it, or sell it back.

You will also earn another 5 or so days between now and then, along with what you earn while on leave.
Most units limit leave to 30 days taken at a time. In order to avoid losing any or selling it back, you need to talk to the folks at the transition office and your CoC.