Why is 'on' being used here after 'back'? Does this 'on' give a specific nuance to the sentence?

What is the meaning of “usual lineup”? Is it something like “regular routine”?

Yes, in this context, "usual lineup" means "regular routine". "Lineup" here refers to the group of chores she has to do every morning.

It seems that "The Little Book of Calm" really exist. Is it considered a well-known book?

"The Little Book of Calm" is a book written by Paul Wilson. It was written to advise people on how to deal with stressful situations.

What does "I guess" mean in this case? Is he guessing his own intention?

"I guess" is an informal expression that is very versatile in use. Generally, it is used to signify agreement, however, it indicates slight reluctance or uncertainty. It is somewhere between a "yes" and "maybe." It can have a similar meaning to "I suppose" or "I think." The speaker has used it here to agree to sleep with her, but is showing his slight reluctance to do so. Ex: He's pretty smart, I guess. Ex: I guess you're right. Ex: A: Don't you think this is a great idea? B: I guess.

What does "why" mean here?

Besides asking for 'reasons' and 'explanations', 'Why' can also be used as a term to express surprise, as used in the fragment above.

Why would Jimmy Kimmel say "regular school" in this context?

Jimmy says "regular school" since, in the world of acting, it's quite common for child actors to be taught through homeschooling or with private tutors instead of "regular school" so that they can do their acting jobs as well. And then he also makes it part of a joke by saying that maybe Johnny Depp taught him instead of going to a "regular school." Ex: I was homeschooled for a few years, and then I went to regular school. Ex: You can go to a dance school instead of a regular school if you want to.