What are “is” and “was” pointing to, or referring to in these sentences?

Question

Frank is my best friend!
Frank was my best friend!

Here, is and was are pointing to the present time, and past time.

Mom: Look, you dirty little grub!
Child: Mum, it wasn't my fault.

What is wasn't is pointing to, or referring to?

Child: Mum, it isn't my fault.

What is isn't pointing to, or referring to?

Can I assume is focuses on attention, while was focuses on event?

MOM: Jack and Hughie got hold of the doll before poor Meggie had a chance to see it properly.
DAD: Well, boys will be boys. Is the damage bad?

or

DAD: Well, boys will be boys. Was the damage bad?

The doll was damaged before the conversation starts.

To what are is and was pointing, in the above sentences?

A: But, Sister, it was my fault!

B: It is a matter of complete indifference to me which one of you was responsible.

or

B: It is a matter of complete indifference to me which one of you is responsible.

To what are is and was pointing, in the above sentences?

One more sentence.

Son: I just wish we'd get rich one of these days, so you could have a maid.

Mom :That is wishful thinking!

or

Mom: That was wishful thinking!

To what are is and was pointing, in the above sentences?

Answer 1

"Is" and "was" in all cases are referring to something in the present, and something in the past, just as in your initial samples; the question really is, how is the situation being considered in each case in order to make present-tense or past-tense appropriate?

When the child says "it wasn't my fault", they are referring to the event in the past that caused them to become dirty. When they say "it isn't my fault", they are referring to their current condition of dirtiness. Which one they use will probably depend on what they think their mom is more upset about, the dirtiness itself or the reason they got dirty.

When the Dad asks "Is the damage bad?", he's focusing on the current state of the doll. When he asks "was the damage bad?", he's focusing on the state of the doll immediately after the boys finished damaging it. "Was" is more appropriate if he expects that the Mom has already taken some action to repair or replace the doll, while "is" is more appropriate if he believes the doll is still damaged.

EDIT to address the added/merged content:

Is still always "points to" the present, and was still always "points to" the past. In the "is/was responsible" case, saying "is responsible" is very unlikely; since the original speaker referred to something in the past ("it was my fault", whatever it was), the respondent would naturally match tense when referring to the same thing.

Similarly with the son/mom, the mom would only say "that is wishful thinking", because the son is currently thinking it; it is a present event and needs to be referred to in the present tense. If the son were to say "I used to wish we would get rich so that you could have a maid", then the mom could reply "That was wishful thinking", because the thinking clearly took place in the past.

About the only thing I can think of to offer for references is dictionary.com's definitions for is and was:

is: 3rd person singular present indicative of be.

was: 1st and 3rd person singular past indicative of be.

Answer 2

(1) & (2) The subject of "is" and "was" is "Frank". Frank is doing something, namely, being your best friend.

(3) The subject is "it". "It was ..." Exactly what "it" refers to is whatever it was that caused the mother to be upset.

(4) "The damage".