Yes, you should not lose your unlimited data plan if you purchase devices outright at full retail. You should be able to purchase your device and then activate it on your plan as noted in a prior post. When you talk to anyone with Verizon, be very clear that you do not want your plan touched!! If you go to a Verizon store, have them print out your receipts, etc. before you leave the store - and read the long receipt to verify everything - instead of just signing their tablet screens and letting them send you an electronic receipt later.
We have found the phone reps to be very helpful and consumer oriented, and found the store reps to be more sales oriented.
Also, after any transaction, addition, etc., verify immediately online via your "My Verizon" that your data plan has not changed. And check frequently to make sure nothing gets changed.
One issue we found with 3G/4G is that our "old" iPad2 does not work on 4G. If all your devices are 4G compatible, you won't have a problem, obviously. But if you have a 3G only device, you'll want to make sure you can specify 3G when you need it. Also, have you considered using your phone as a hotspot rather than (or in addition to) the mifi? On the iphone6, you have 4G and also are able to restrict the wifi signal to 3G if needed.
I may be stating the obvious, but data intensive apps, entertainment options, and websites will only get more data intensive in the future; hold onto and protect your unlimited data plan like a dear, dear friend.
