And, the reason the numbering is an issue is that the canon says that a Time Lord gets 12 regenerations, and so 13 incarnations. Inserting Hurt means that the guy stepping in after Matt Smith leaves will be the "last" Doctor.
What it actually means is that, somewhere in his tenure, the new guy will have to find an excuse for more regenerations, which can be a cool story.
Yes and Yes.
I never assumed the 12 regen limit was something they'd stick to, more like something the writers declared in the old series when they thought they'd never hit the limit (kind of like using 2 digits to store the year in a database).
As such, now that the 12 regen limit is looming, if Dr. Who is popular, they'll come up with a way to bypass it. if it is waning, they'll use it as a way to finally end the series. (until 10 years later, when somebody tries to revamp/reboot Dr. Who)