Pertaining to unlocking you at least need a wifi connection on the computer doing the unlock from my experience, but not active Sprint Service.

For the question about using two lines with one number to have an active DATA line at all times....The cloning problem was already addressed. But the ddtm is a very great suggestion. (If you can get Sprint to give yout the service (NO talk) for $20 something a month basically it is unlimited text and data only. And would suit your needs perfectly.) Better use than a mifi and it can be used a spare battery charger