Theory:

     “Fact Number 1,” I read, “his name is Mr Nath. We must discover his first name.”

     “Do you think that’s his real name, Maya?” Nishad asked.

     “Probably not,” I said. “Most crooks have an alias.” I added a big question mark after Nath.

     “Fact Number 2,” I read on, “the tenants at Shankar House say he’s mad, strange and unfriendly.

     “Number 3, he doesn’t talk to anyone and is mannerless.”

     “But he did talk to us, Maya, and Mamma says he’s very polite,” Nishad interrupted.

     “He only talked to us because he had to,” I said, “and since he was under Mamma’s medical treatment, he had to be polite.
Explanation:

Maya began to recite the facts she had compiled. The first fact was their suspect's name: Mr Nath. However, they knew that Nath was a surname, and as a result, they wanted to figure out what his first name was.
 
Nishad wondered aloud whether Mr Nath was the suspect's true name, and Maya explained that as most criminals use a fictitious identity, their suspect must have used one too. She then adds a question mark beside the name, turning the statement- "his name is Mr Nath" into a question, as in, ‘Was it his name?’.
 
The second fact was that the tenants at Shankar House described their suspect as mad, weird, and unfriendly.
 
The third was that Mr Nath was rude and didn't speak to anyone. However, Nishad disagreed with this one because he reminded Maya that he had spoken to them, and that their mother had said that he was courteous.

On the other hand, Maya informed him that Mr Nath only spoke to them because he had to and that he had to be courteous since he was being treated for his scars by their mother.
  
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Was Mr Nath polite or rude?
 
Meanings of difficult words from the paragraphs:
  
Sl. No.
Words
Meanings
1
AliasA false or assumed name or identity
2
TenantA person who occupies land or property rented from a landlord
3
PoliteHaving or showing behaviour that is respectful and considerate of other people
Reference:
National Council of Educational Research and Training (2007). Honeycomb. Expert Detectives:Sharada Dwivedi (pp. 126-132). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.