I really need some advise and help with this one because it seems all my explanations and what my co-worker is reading in the grammar books is not satisfying her!!!

I want to know if the normal rules for changing direct to indirect speech apply within Q&A format and which is more common of the two I mention below?

The situation:

she is making self-study material for non-native learners who will not have access to a native speaker when doing the material.
The Q&A are written so that the answer is partially written and the students have to fill in the gap with info from the story - exactly as it appears in the story... so the Q&A are all very controlled etc...... (don't ask!)

so she is using the story Matilda by Roald Dahl. This is an excerpt....

"So I buy an old dump that's got about a hundred and fifty thousand miles on the clock. I get it cheap. But no one's going to buy it with a mileage like that, are they? And these days you can't just take the speedometer out and fiddle the numbers back like you used to ten years ago. They’ve fixed it so it’s impossible to tamper with it unless you’re a ruddy watchmaker or something....

First Q&A - options
1. What kind of car did Matilda’s father say he would buy?
Matilda’s father said he would buy old cars that have about a hundred andfifty.....

2. What kind of car did Matild’s father say he buys?
Matilda’s father said he buys old cars....

Second Q&A – she doesn’t know if she should use ‘is’ or ‘was’.
1. What did Matilda’s father say was impossible to do with the speedometer.
Matilda’s father said that it was impossible to ....

Some info from the grammar books she has given me:

1. It is not always necessary to change the verb when you use reported speech. If you report something and it is still true, you do not need to change the verb.
Note – that it is also correct to change the verb into the past.

2. Sometimes the present tense is retained even in formal English when the reported sentence deals with a general truth.

3. Whatever the tense of your reporting verb, you put the verb in the reported clause into a tense that is appropriate at the time that you are speaking.

However, when the reporting verb is in a past tense, a past tense is also usually used for the verb in the reported clause even if the reported situation still exists. For example, you could say ‘I told him I was eighteen’ even if you are still eighteen. You are concentrating on the situation at the past time that you are talking about.

A present tense is sometimes used instead, to emphasize that the situation still exists.

Ok – so all this info form the grammar books supports what I have said to her but she still is not satisfied. She wants to know if it is ok to always put into the past – or when should we keep the present. She seems to want to create her own little rule.

Is one use more common than the other?
Can you recommend when you would use on than the other or is it just a preference thing?

Any help is really appreciated!!!!