"Were to have" vs. "had" / conditionals in reported speech

Answer to questions 1 and 2

A native speaker might say either, but both sentences have different meanings.

“If I had a helping hand … .” You state that you consider a helping hand necessary, and hope that someone will offer to lend a hand, or assistance will be allocated. Someone might help.

“If I were to have a helping hand … .” means you are imagining a situation whereby you will get the helping hand you need, but you've never had an assistant and you don't expect to get one. You won’t get help and you're not really looking for it.

Answer to question 3

(1) Paul said that if he had had a helping hand, he wouldn't have had to work overtime that day.

Or you could say:

Paul said that if he'd had a helping hand, he wouldn't've had to work overtime that day.

1) You would probably say:

“If I had a helping hand, I wouldn’t have to work overtime today.”

2) It’s complicated. If you use had had (not a mistake, there should be two) you get a past meaning of having an assistant, and instead are saying if you had had an assistant in the past, you wouldn’t have to work overtime now/later.

“If I had had (in the past) a helping hand, I wouldn’t have to work overtime today.”

“If I’d had a helping hand, I wouldn’t have to work overtime today.”

Were to have is more focused on the future, or a wish or if only:

“If I were to have a helping hand, I wouldn’t have to work overtime today.” Expresses a similar meaning to:

“If only I could have a helping hand, I wouldn’t have to work overtime today.”

Had on its own expressed a present meaning.

“If I had (right now) a helping hand, I wouldn’t have to work overtime today.”