Pub 519 says:
Foreign tax credit.
If you receive foreign source income that is effectively connected with a trade or business in the United States, you can claim a credit for any income taxes paid or accrued to any foreign country or U.S. possession on that income. If you do not have foreign source income effectively connected with a U.S. trade or business, you cannot claim credits against your U.S. tax for taxes paid or accrued to a foreign country or U.S. possession.
Because you have determined that all your income is NEC (not effectively connected), there's no foreign tax credit. But your home country may offer one to combat double taxation. (Think about this: If US also allows a foreign tax credit in your situation, won't the credit be claimed twice?)
That being said, I don't think the dividend reported on 1099-DIV is US source income only, because all dividends, regardless of the source, are taxable for the US for residents. 1042-S has income code that distinguishes "Dividends paid by U.S. corporations—general" and "Dividends paid by foreign corporations". Since your income is NEC, the former one would be used for US tax return, and both may be used for your home country tax return.
I don't know whether US-based mutual funds that trade foreign stocks are considered to generate US source income only, but I would view this with a grain of salt because mutual funds are passthrough entities. At the end of the day, you may be overpaying with a 1099-DIV.
Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or tax advice.
