What do you call a person who is guided by someone or something else in a mentor/assistent manner?

Consider guidee, which means

one who is guided.

An example sentence:

"The guidee was suddenly stopped by the guide."

It doesn't fit your desired context, though. For that, you might just want to go with a higher sortal, like "person":

"A person can only receive guidance from 1 ancestor at a time."

In fact, higher sortals work in all your examples, and are, in my opinion, preferable to using a synonym of "the guided" or "follower". Using the latter in your contexts actually creates redundancies since the consequent material of all of the sentences make it plain that the contexts are ones of guidance. There is no use to encode this information twice.

"The tourist was suddenly stopped by the tour guide".

"The blind man was accompanied by his guide animal."

"The executive was on a tour of the building he helped fund."

All of these sentences contain more information than would the corresponding sentence where the italicized sortal is substitued with a redundant synyonym of "follower".

I would use the word protégé:

A person who is guided and supported by an older and more experienced person or mentor.

I think it fits really well in the context of your book, since you are looking for a word for someone who is mentored. In any case, ancestors are, by definition, older if not more experienced.

This word may not fit the other contexts you mentioned, but I think that the examples put too much emphasis on the "assisting" part of things.

You can also use mentee.

A person who is advised, trained, or counselled by a mentor.

For example,

"The mentee received constant guidance from her mentor."

Good luck!