Would it sound awkward if I say "single" instead of "one" in this context?

Isn't it right to say "didn't you" not "did we" considering the meaning? I'm curious why he used positive which is "did" and "we" (but it seems like asking to Ron alone).

It is actually acceptable to use "we" sometimes in place of you to address a second party. And so here "we" is in reference to a singular person, Ron who is alone. In British English sometimes we use the positive as a sarcastic query for what is an obvious answer. Here obviously Ron had a late-night snack, so the use of the positive "did we" is sarcasm. Ex: How are we feeling today? Ex: What are we in the mood for?

What does "filed by" mean?

Excellent question. "Filed by" here is a legal term used to mean to start a court action. In this example, you could replace the term "filed by" with "started by" and the meaning of the sentence would remain the same. The word filed is used because a written complaint or petition (a file) has to be given to the court as the opening steps in a lawsuit. This creates the base of the "case file" which all other documents to do with the lawsuit go into. Ex: When are you going to file the complaint. Ex: The answer will be filed tomorrow.

Does the phrase "bundle of joy" only refers to a child? Or can I use it to whatever precious?

The expression "bundle of joy" is only used to refer to babies. Ex: My sister and her husband recently welcomed home their bundle of joy. Ex: The couple came home with their new bundle of joy.

Would it sound awkward if I say "single" instead of "one" in this context?

It wouldn't sound awkward, no! You could definitely replace "one" with "single" here. "Singular" would actually sound a little bit better here! "Singular" refers to just one person or thing. Ex: There's no singular right way to interview and ask questions. Ex: She's the singular person that I trust at the moment. Ex: There's not a single reason why I want to stay here. = There's not one reason why I want to stay here.

I think there is no verb in this sentence. If I put the verb, I should put "were" after "owners". Am I correct?

Yes, there is no verb in this sentence. Speaker is skipping the verb "were" here. If you make it a grammatically correct sentence, then you should say "Cat owners were the most dependable and emotionally sensitive."