@investng engineer, Thanks for your answer.
The W8BEN is indeed a separate matter: it only affects withholding, not how much tax I need to pay to the IRS.
As far as I know, the dividends reported either in 1099-DIV or in 1042-S are all US sourced income (since they originate in US-domiciled mutual funds), and therefore are taxed by the US. In my case they are considered "not effectively connected income". As such they are reported on Schedule NEC, and the tax rate is according to the tax treaty between the two countries.
Per my understanding, the foreign tax is paid/withheld directly at the mutual fund level, and not by the broker, so it is already deducted whether I receive 1099-DIV or 1042-S, and whether there is broker withholding due to W8BEN or not.
Given that tax on the dividends, which are all US sourced income, is owed to the IRS in any case, I was wondering in my original question if the foreign tax that was already paid can be used to offset somewhat the US tax owed.
