Well, I don't know about the requirement to purchase from Verizon, but I do know that you DO NOT need to buy on Edge to get the $25 credit.
I recently bought 5 refurbs from Verizon, NOT on Edge, and I do get the $25/mo credit for all 5 lines, so $125/mo total.
Of course, I will caveat by saying they may have since changed their policy. I made the purchase in October 2014.