Does Plan B's B stand for a "back-up?"

Is it okay not to put "a" in front of "little man"?

Yes, it is okay to not have an article in front of "little man" here because "little man" is used as a nickname in this case and so it is a proper noun. Names and other proper nouns generally do not have articles like "a" or "the" in front of them.

Is there a significant difference between "germ" and "virus"?

No, there isn't a significant difference. Actually, "germs" can be a "virus." "Germs" are viruses, bacteria, fungi, and protozoa that can cause us to get sick. A "virus" is a specific type of germ that needs another living host's cells to multiply/reproduce. Ex: We don't know much about the coronavirus. Ex: Wash the counters after cooking to kill any germs. Ex: The virus had me coughing for 3 weeks! Ex: She is such a germaphobe.

What does "as a plan B" mean? Can I use other initials instead of B?

"Plan B" is used to mean an alternative plan of action for use if the original plan fails. You could use any letter but generally "b" is used because you're referring to a second plan. "Plan A" refers to the original plan and any other letter would have the nuance that it is the same number plan as that letter in the alphabet. Ex: A: Well, there's always Plan B. And if that fails, Plan C and D. B. How many plans do you have? A. I'm covered all the way up to G. Ex: Let's just go with Plan B.

Does "inhabit the role" mean "take the role"? What does it mean?

Great question. Usually, when someone "inhabits" a place, they live there. You can inhabit an actual place, like a home, a cave, or a neighborhood. You can also inhabit an imaginary world, like making up a personality/life online, talking about experiences that are nothing like your real life. So when we say that actors "inhabit their roles", it means that they seem to become the character that they are playing. They are no longer just actors reciting their lines. It is like they live the life of the character. Ex: The people inhabited the islands that are now deserted. Ex: He really inhabited the character. I forgot he wasn't really a doctor.

What does "went into" mean here? Can I say "went through" instead?

"Went into" or to "go into" in this case means that something was included or done when doing an activity. So "a lot of thought that went into this" means that she thought a lot about that outfit and spent a lot of time thinking about how it should be put together. It wouldn't be "went through" here, because when describing how something is applied such as "thought" in this case, the preposition should be "into". Ex: This cake may not look pretty, but a lot of love went into making it. Ex: A lot of work goes into making this art.