A jailbreak will do nothing except what it normally does. Software unlocks have not been useable for a couple of years now.I am unsure if a jailbreak would help you on your way, but then you'd have to be willing to do that if it does.
All your other information is totally correct. Somehow, I see the iPhone 6 being excluded by Sprint. If it comes out in September then that is before January/February 2015 which means Sprint can lock it down.
BTW, Sprint updated their unlock policy a few months ago so that any blame on this is cast on the manufacturers and not on their patent refusal to unlock. I speculate that they rewrote it this way because they anticipate a flood of unlock requests. So, blame it on the manufacturers and not Sprint. Nevermind that it's Sprint that has dictated this.