You won't be able to activate you Echo on anything but an Everything Data or Simply Everything Plan. You'll still have to pay the $10/mo smartphone fee. You certainly won't get it on your $42/month plan. The only options without changing plans are either old smart phones (like the TP2) which aren't systematically enforced to have the $10/mo fee (though, they technically are supposed to have it applied) or get a dumb phone.
You made an awful long post about something that won't even be possible from what I've read
. You can check out sprintusers.com forums and probably learn more.