> Would it be possible for me to get American clients just through email?
Yes. Also your English is fine, assuming this is a representative sample.
> considering the present reality of code-on-whiteboard sort of interviews for a stable job in the major software companies
My advice here is that it's a coder's market and you can mostly redirect these into other methods of demonstrating your competency, or if nothing else - iterate over several companies until you find one with more realistic methods.
> What do you think about passive income, say, from selling WordPress themes on themeforest, or something similar?
"Passive" income is mostly a misnomer. It's more a matter of, "I have a mostly post-development product, which occasionally requires maintenance, and the active portion of monetizing it is now more a matter of sales and marketing, versus engineering."
More generally:
The set of companies hiring or expecting to hire software engineers is approximately, "all companies which presently employ software engineers." The set of companies which can benefit from software engineering is approximately, "all companies with meaningful revenue."
The part that remains is for you to find them and convince them that particular things you can do with code will predictably lead to them having more revenue. This is easier if you specialize in a particular type of company and problem, because you can build up knowledge around what problems they have, how they currently deal with them, how they talk about them, and what sales and marketing approaches they're most receptive to.
This applies whether you want to be an employee, or sell a product, or provide consulting services, or whatever. Find people who have problems, convince them that you can provide solutions in a way which is predictably revenue-positive and which doesn't unduly increase the burden to them.