It is impossible to give you correct answers as we don't know where your money is invested. You would have to look at the Double Tax Treaties of each country.
Some countries have a Withholding Tax on Dividends and Interest and some don't. The UK for instance does not and the US does.
You would also have to read up in the DTA if the Dividends Withholding Tax is capped at a certain percentage. Most are capped at 15%. Chances are that your foreign broker will not apply this cap is a given. You have to inform them yourself.
If you are invested in the US then it would get complicated. The US does not have the normal 10 page Tax Agreements but they have a pdf book that instructs other countries on how to treat them and how they will reciprocate in cross border taxation. I have no intention to spend a whole day reading it as it does not concern me. Patrick will have some input for you as he has investments in the US.
You can use the yearly average price or the closing price on the day the money is made available to you. Here is a link to the European Central Bank for prices on any particular day.
https://www.bportugal.pt/conversor-moeda