I'll answer your first question, and then perhaps someone else will look at the others.
With reported speech, there is indeed room for ambiguity, as you imply.
1. Bill says: "I never eat meat".
2. Bill said he never eats meat.
3. Bill said he never ate meat.
#3 is the standard reported form of #1. The "pastness" of the reporting clause doesn't reflect the "pastness" of the action ("never eating meat"); it reflects the "pastness" of the time of utterance. The verb in the reporting clause ("ate") is simply in accord with the verb ("said") in the main clause. Therefore it isn't possible for the listener to know whether Bill now eats meat or not. However, if it does relate to the literal past, it's common to add clarification, e.g.
4. Bill said he never ate meat in those days.
(At other times, context will make all clear. For instance, #4 might continue: "Though now, of course, you rarely find him without a beefburger in his hand.")
The #2 form is also very common, where the action in the reporting clause is still current, or where it relates to a general truth. Indeed, some native speakers are very uncomfortable with a past-tense verb in the reporting clause, in such cases. You will even sometimes come across a native speaker (or grammarian) who insists that this kind of sentence is ungrammatical:
5. When I was a child, I didn't believe that the earth moved round the sun.
and should be changed to this:
6. When I was a child, I didn't believe that the earth moves round the sun.
on the grounds that the earth still moves round the sun.
However, if we are to judge by the practice of significant writers of English, both forms are perfectly acceptable.
Let me know if any of the above is unclear!
All the best,
Student or Learner