whats the difference between pendant and torsque?
Pendant means during or while. Lorsque means when.
Quand also means when and often the two are interchangeable. However there are some unique uses for each and if you want more detail, you can learn more here: https://www.thoughtco.com/quand-in-french-1368933
Why not "He is reading while eating."?
That is correct!
Why is "qu" in this sentence?
« Pendant que/qu' » as a whole is a conjunction meaning "while." It connects two parts of a sentence together.
can this be used to talk about a single person and used in the same way to talk about two people?
It's possible that this could be about one person or two, context would tell. If the pronouns are the same, it typically is assumed to be the same person, but lacking context, most speakers would use a proper noun instead of "il".