So I have a theory about Cowboy dude’s way of appearing immortal and his role with the Nomicon.
Let me offer some counter arguments here:
1. Bruce is a time traveler who goes back in time with info from the future as to who the next ninja will be for past self to look after.
This isn’t possible, since what would happen is this: Bruce from past travels to future to watch over Ninjas, disappearing from present. After a certain amount of time, future Bruce travels back in time with specific dates for a Bruce that existed in time slightly before the now-future Bruce originally time hopped to the future. However, if future Bruce stayed in the present of past Bruce, while new-past Bruce goes to the future, there will be no old-past Bruce to go to the future the first time to go back in time to tell new-past Bruce to go to the future. Therefor the whole series of events never happened in the first place and now we are back to one Bruce travelling to the future with knowledge of Ninjas that may not come to be because the first Bruce didn’t travel to the earlier part in the future to raise the Ninjas that came before these newer ninjas. TIME TRAVEL IS CONFUSING. Also, all the while, Bruce is aging still, so that would begin to show, even if he did jump to the future.
2. Bruce is actually the human!Nomicon.
Nice as a headcanon, improbable as a truth of the show. First point: that the Nomicon splits its time between being Bruce and the book, is highly unlikely because we see that Bruce has a life and relationships outside of school and most certainly outside of Randy. He isn’t just jumping between the two constantly. Your second point is that this theory is likely because Bruce time travels and the Nomicon knows the future, but that assumption is incorrect. The Nomicon is drawing on personal experiences from the past. All of its knowledge is due to fighting the same immortal foes over and over again, and having 800 years to understand the nature of humanity. In fact, the Nomicon has been proven wrong in some cases, and if it knew the future, it would know a way to word its messages so that Randy could understand them…
Your third point ties into your previous points, too, and while I do think its likely that Bruce played a heavy role in the making of the Nomicon and guiding the Ninjas, I still think it is more likely that he is immortal, than simply time travelling (though a time travelling immortal is also possible).