1. Because There Is Logistically No Reason Not To
Sure, the Comic Con footage (reportedly) showed John Hurt's Doctor in the Time War. Sure, John Nathan Turner deliberately had that line of dialogue changed to the ridiculously vague 'somewhere between your twelfth and final incarnation specifically to avoid us ever having to have this conversation. Those two things actually make this a brilliant twist. Say he WAS Doctor 8 1/2. And Clara was part of that time. And somehow she got caught up in that event to an extent that it caused that version to be brought forward to become the Doctor again when Matt Smith died. Not only could it be worked to seem plausible - it could also explain why Clara doesn't remember him from the timeline (her own Timeline was too intermingled with the distortion) and why they worded which regeneration he was so vaguely (well... he's sort of the 8 1/2 the version of you that got usurped into somewhere around your eleventh regeneration...) It all sort of works. I am ready for an anti-hero Doctor who spend a few years doing bad things before he learns how not to do bad things again. I think that would be amazing.