indicative mood and subjunctive mood in indirect speech

1. If I come into a fortune, I'll give up working.
2. I said that if I come into a fortune, I'd give up working.
3. If I came into a fortune, I'd give up working.
4. I said that if I had come into a fortune, I'd give up working.
Source: Ting-Chi Tang, A Guide to Teaching Junior High School English, p348.

Can I use "came" instead of "come" in #2?
Can I use "should" instead of "had" in #4?

sitifan said:

Can I use "came" instead of "come" in #2?
Can I use "should" instead of "had" in #4?

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Not only can you, but in my humble opinion you should (in both cases).

3. If I came into a fortune, I'd give up working.
5. If I should come into a fortune, I'd give up working.
6. If I should come into a fortune, I'll give up working.

What's the difference in meaning between the above sentences?

I don't see any difference. (Perhaps others will disagree.)

sitifan said:

4. I said that if I had come into a fortune, I'd give up working.

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I'd word this as "I said that if I had come into a fortune, I'd have given up working".