I would clarify your order by saying "we would like one order of spare ribs" or "we would like an order of spare ribs to share/split." Grammatically you are correct with "one spare ribs to share," it just sounds funny to ask for "one" of some thing that is served in quantities of more than one. But, in the end, you would still end up with a single order of spare ribs, so I think you are fine with any of these.

No, it is not correct. It should be one portion or one plate of ribs.

I wouldn't say it this way, but it's a shortening of "we'd like one order of spare ribs to share." Meaning one portion to share between the people.