For the recouping of withholding taxes...
You get a T3 slip at end of the year from brokerage. Looking at mine, in the "other information" there was info for box 34, which is "Foreign non-business income tax paid". This goes into form T2209 and ends up coming straight off of your federal tax owing. I THINK that is how it works.
The $100,000 I mentioned is just if you own assets directly in the US, so a ETF listed on a US stock exchange like VTI would count towards that. But this just means you would have to fill out form T1135. I've never done it myself, but don't think it's actually a big deal, or has any tax implications.
I don't know how you would get a 10K tax bill if you only made a few hundred dollars in dividends and didn't sell anything. That 10K is just from your investments? Did you accountant explain it to you? Did you have some unusual ETFs? I don't know, did you have 2 million dollars in a high interest savings account? Your T3 and T5s have all the relevant information for investment income, any big numbers on those?