A year too late, but what did you end up doing for figuring this out?
Regarding #2 and #3 ...
I am not an expert and cannot give formal advice, but my understanding is this:
- The amount of income realized from RSU grants is the market value on the date of vest
- The portion of that income is split between countries based on the number of days between grant and vest spent in each country
- Some foreign payroll systems (e.g. tech companies like Microsoft and Google) do not support or decline to implement reduced withholding in Canada to account for claiming of foreign tax credits
- As this is a payroll function, filing forms with a brokerage probably won't help
- RSU income is reported as full wage income in both the US and Canada and is later claimed back using foreign tax credits
- Canadian and US returns require numbers from each other for computing foreign tax credits to avoid double taxation
- The US allows claiming "accrued" taxes (owed but perhaps not yet paid), and this allows bootstrapping the process between Canadian and US returns
- Any provincial foreign tax credit appears to be 0 if the full amount of foreign taxes paid is deductible on federal taxes (my understanding of CRA form T2036) [1]
Because of #8, the extra withholding for provincial taxes--which appear to be double-tax foreign-sourced RSU income--helps keep you "on the right side" of tax liability, paying too much tax rather than too little.
It's also worth noting that truing-up actual taxes paid after any negotiating, adjustments, and actual filings needs to be done for any "substantial" difference from original numbers. For the US I believe this is amending your return to match up actual Canadian taxes paid relative to the estimate used during initial filing for Canadian taxes estimated to have "accrued" up until that filing.
[1] from CRA form T2036: "If the amount of the federal foreign non-business income tax credit you are entitled to deduct is equal to the foreign non-business tax you paid, your provincial or territorial foreign tax credit would be zero."
