A bit confused...

Thanks for your reply.
Do you have an exemple to illustrate in which case you'd use "orange county residents" versus "orange county's residents" ?
Anyone for points 3 and 4 ? I think point 4 has something to do with subjunctive mood correct ?

bump
3) Which version is correct in this sentence : John played football with Holmes who/whom scored a touchdown ? (is it correct at all ?)
4) Do you say "suppose it were to happen" or "suppose it was..." ?

Anyone please ?

3) "who"
4) "were", because this is the classical hypothetical subjunctive.

1) Would you say "orange county residents" or "orange county's residents" ?

You'd normally say the former. However, the latter is not grammatically incorrect. Here's a good sentence for each:

Orange County residents are protesting a new ruling that would increase their energy expenses.

Practical jokesters managed to fool some of Orange County's most prominent residents.
(Because of the separation between "OC" & "residents," only the latter construction makes sense here. By the way, here's the amusing factual basis for my sentence: https://www.nbcnews.com/id/4534017/ns/te ... O9n8mh4rYg)

2) What's the right idiom : "X is native to Y" or "X is native from Y" or "X is native of" ? (i know i can rule out the second one, but i'm having doubts about the other two)

The first is correct. The third will work if you add an article, making "native" a noun.
This fish is native to Alaska.
Brenda is a native of Alaska.
In the first sentence, "native" is an adjective, while in the second it's a noun.

3) Which version is correct in this sentence : John played football with Holmes who/whom scored a touchdown ? (is it correct at all ?)

Throw a comma before "who" and you've got a good sentence. You choose "who" or "whom" based on whether the person referred to is serving as a subject or object in that part of the sentence. In other words, the fact that John threw the ball to Holmes doesn't permanently designate Holmes as an object. He's the one who scored, so he's "who," not "whom." Here's another example:

Phil has contempt for anyone who thinks that the current plan is working.

Notice that while Phil has contempt for "anyone," since "anyone" is doing the thinking, we still say "who."

Meredith is reading a book about Mary Oliver, whom I just interviewed last week.
Here, Mary Oliver is getting interviewed, so "whom" applies.

4) Do you say "suppose it were to happen" or "suppose it was..." ?

If we're creating a conditional, we use the former. If we're speculating about the past, we use the latter.

I don't think we'd see "suppose it were" too often on the GMAT because it tends to be used in less formal constructions:

Suppose war were to break out. What would you do then?

I suppose I could see "suppose it was" showing up:

His offer was rejected by the committee, who supposed that it was just a ruse to gain more time.