I am a Pakistani national, legally residing in Saudi Arabia. Can I apply for an E2 visa from Saudi Arabia or try from Pakistan?

Usually a U.S. consulate will process visas for anyone legally residing in the country where it is located. However, since Saudi Arabia is not a "treat investor" country, you may want to consult with an American law firm that does business in Saudi to find out how these are best handled.

If you are residing in Saudi Arabia, you can probably process for the visa there. You need to retain an experienced immigration lawyer, whether myself or one of my colleagues, to review all the facts, advise you, and handle the case.

If your wife is a citizen of Pakistan and wishes to apply for a treaty investor visa, she can.

Most likely yes.