Yes! You remembered Oshino's charm from the alternate timeline. And you are correct, that's the reason the First isn't there. So let's go through a little timeline with some extra novel info:
-We know that Shonibu's kind attracts oddities, because as aberration slaying vampires, that's one way they have to feed;
-The first kin had strong ties to that city, since it was his hometown, so his ashes were "led" there;
-15 years ago he arrived there, this brought the weakened god of the shrine to ruin and reestablished the air pocket where oddities gather in the city;
-In the novel, Gaen repeats to us that someone who meets an aberration is more prone to meeting them again, but adds that even so, Araragi shouldn't be encountering them every other day. That's the connection Gaen mentioned, the First only made it a little easier for aberrational phenomena to take place there. Just the difference of a five percent chance turning into six percent, or seven percent at most;
-It doesn't seem like much, but she said that’s significant from an expert’s point of view, life gets hard for them once you’re past the five percent mark;
-So we can assume things like the Phoenix wouldn't lay their eggs there without the extra 2%;
Now let's think of some more recent events, and also talk about the time-travel:
-Gaen told us that his recovering state was progressing very slowly, and then came Meme with the talisman he gave to Araragi in Bake, and said talisman swiped the energy that could potentially serve the First’s ashes, delaying his resurection even more;
-But then came a very important event: Araragi wanting to go back in time;
-Shinobu used all spiritual energy at the shrine to open the gate, but she did something for him much better than feeding: she came close to him. Since Bake it is well established to us that Shinobu can't do much while far away from Araragi. He told the Cat when searching for her that she can't use her abilities unless they are close, and also during Neko Shiro, Shinobu couldn't even fly to the cram school while their pairing was severed, hence the Cat gave her a lift.
Coming close to the First was all she needed to do to boost his regenerative skills. After that, it took less than a week for him to get from ashes to this point with a whole body;
Their blood link brings them closer. The First was slowly getting his form back, so during spring break in Kizu, Kiss-Shot was lured to this city once again, even if she was not aware of it. This is also what made the whole Gaen Network start acting, as she stated in this episode.
But what about the Route X world, why didn't the First get back to life there? After all, not only did Shinobu also come to the town, but also regained her full powers back. Having such a powerful closely related vampire so close to oneself should speed things up as well.
That's where Meme's talisman comes into play.
When they were in the parallel world they had a conversation about the talisman that didn't make a lot of difference to us at the time: in the original timeline, Meme gave Araragi a talisman that dispersed spiritual energy, but in Route X he gave them a talisman that _absorbed_ spiritual energy. Both had virtually the same results, but the second one was several orders of magnitude stronger. Koyomi and Shinobu couldn't even touch it without getting hurt.
So with that we have a reason for the First not to have revived in Route X: he was no more, completely exorcised by the talisman. Even if not absorbed by it himself, his food was completely destroyed instead of just dispersed.
This arc is basically the pay off of a lot of previous foreshadowing and the final episodes are just top notch!