I would go ahead and return it now; be sure to reverse and save your upgrade. Otherwise your 14 days are gone and you won't get another 14 days with any replacement. In other words, whatever you get to replace the One X+ will be yours for the next two years, no 14 day grace period. Return and reverse means you don't get a different or replacement phone, you go back to your old one. You will have to start from scratch at some point in the future when you are ready to choose another phone.
But, doing it this way can cause two problems: you will likely be charged a restocking fee ($35) and; they won't refund the upgrade fee ($36) you already paid - that means you will have to pay it again with the next upgrade. This will make your next phone actually cost $71 more than what you will pay for it.
Here is how to fix both problems. As you are reading this right now, call 800-331-0500 (but keep reading); after confirming your number and ID, press "0" then press it again. This will take you to Customer Care. Be POLITE, DON'T GET ANGRY (they are there to help you and you want them to sympothize with you) but be firm. Explain the problem and that you want to return the phone. There is a design flaw that HTC is aware of and is working on, but who knows how long it will take.
Tell them how disappointed you are in the fees policy and that you are being charged for a problem / condition that is not your fault; after all, you LOVE the phone and really want to keep it. If only the BT worked properly, because you use your BT most of the time you are on the phone, and your state law (probably) requires hands-free devices. You dont want some other phone no matter what is offered, you want the One X+!
Ask if there is any way they can waive or credit the restocking fee, and if they can credit back the first upgrade fee. If they will do only one, ask nicely if they would notate your account to waive the next upgrade fee that you will be charged when HTC issues the fix and you re-purchase the phone. Who knows, if you do everything right, are very polite and get a nice representative, you may get all three.
Call a couple of times if necessary, build your claim file, even if you have already returned the phone to the store. If you don't get the credits on the first call, the second one might take care of it. Have you already called ATT Support / Tech to report the problems? If not, do it first and start the complaint file. All of the notes are in the same place and can be read by all of the ATT personnel, even those in the corporate stores, so be nice; if you are a jerk to one of the reps, the notes will reflect it.
If you have done everything possible to report and try to fix the issue first, they are more likely to give you back your $$, as opposed to someone who sounds like they just changed their mind about the phone and now wants to avoid the fees (which is the primary reason the fees are charged in the first place). ALWAYS, ALWAYS, ALWAYS BE POLITE - it works.