[Q] Can I meet the "marginality" requirement by showing I have a lot of assets or incomes from other sources than E-2 biz?

You can try, but the consular officer or USCIS examiner is not obligated to take that into account in arriving at his or her decision on the "marginality" requirement. Why? That's because long time ago, DOS changed the marginality FAM section to disallow the applicant's wealth aside from the E-2 enterprise to meet the marginality requirement.

Having said this, some people might not be aware of this, so you can try to use this fact, but don't go overboard and state that your client-investor is the Bill Gates.