Hi Sam,
I have to say that all this confusion is mostly because of all the fixation on withdrawal of application.
My advice is, ignore the fact that you were given the chance to withdraw the application. It simply means you were not removed, and the removal bar does not apply to you, and you don't need I-212. John brought up examples of his own clients where they were granted the withdrawal of application because of their overstay inadmissibility, and still required (I-192) waivers to waive the overstay bar.
You (and only you) know how long you were in the US, and whether or not you overstayed.
Your last trip did not result in the 10 year bar because you only overstayed for 14 days. The trip that ended in December 20 2014, however, DID.
It all boils down to this and it's very simple: if you have ever overstayed over 180 days, you got the bar and you would need a waiver (or as Ken said, wait out the bar until it expires); if you did not overstay over 180 days, then you did not have the bar (and got charged by mistake, which, with some help, you MAY be able to overcome).
Now you might wonder why was it that they let you in on December 31 if you had been banned from December 20. The answer is, sometimes CBP makes mistakes when determining someone's inadmissibility, and let people in by mistake.
It does not mean, however, that they have forgiven the underlying ground for your inadmissibility, and they are still well within their rights to enforce it the next time you seek admission.
The reverse is also true. Sometimes, CBP mistakenly deems someone as inadmissible, even though they are not. In this case, you may be able to fight your case, though it's not wise to do so alone unless you have significant knowledge in US immigration (and sometimes criminal) law.
Again, it all boils down to this: you either overstayed for over 180 days, or you did not. If you did, then the bar applies.