We have no plans to do stuff with Tilea, but we might do stuff with the Border Princes (as part of the dwarven efforts to make things stable there) and the Border Princes nexuses connect to Tilea. And it's possible that Tilea also connects to the Bretonnia, though I think it probably flows from Tilea to Bretonnia if it's connected at all.
Regarding the Empire protecting Estalian cities, saying "it was Ulthuan's job to save us" isn't really going to make us less dead. Now, as Mathilde herself said to the Empire during the social turn, Ulthuan will probably take care of that if it comes to it, but it's not impossible that something drastic happens and Ulthuan gets distracted - say, a massive Druchii invasion. That might sound far-fetched, but a little bird just told us that things could be getting hectic in the not too distant future.
Yes we might have an action in a place that it next to Tilea in the near future, we also will have actions in the Empire this turn which is next to Bretonia, but that does not seem like it is enough reason to take Mapping Bretonia ASAP, even though we have more connections in Bretonia than Tilea.
As for Ulthuan and restoring/protecting nexuses, it is literally the thing it has been doing for the last four thousand years. I do not think there are such dire signs that they are about to fail at it that the empire, mess that it is, with more beastmen and greenskins in its borders than humans, several provinces that have the Chaos background of the Steppes, needs to step in.