If you decide to be non-resident of Canada, I believe you cannot use a brokerage account, contribute to RRSPs etc. It's not just Scotia iTrade or whatever. It's nationwide.
As a non-resident of Canada, you would file and pay taxes to Mexico not Canada on all your income. Mexico taxes world income. As a resident of Mexico, you could open a brokerage account in Mexico and trade that way.
I assume, however, that you want to keep your money in the stable Canadian system as opposed to the volatile Mexican peso?
In that case, I would look into being a "factual resident of Canada for tax purposes" which means you would file and pay taxes to Canada and maintain a Canadian address on your bank accounts. In that way, perhaps you could keep your brokerage accounts active in Canada.
Another alternative is to designate your Canadian bank accounts as non-resident accounts. You can keep your money in your Canadian non-resident bank accounts, but you still won't be able to trade with a brokerage account. If you don't want your brokerage account in Mexico, you could look at getting an expat brokerage account like Internaxx in Luxembourg. The other option is that you can get a brokerage account in Canada with TD as a non-resident, but you have to show up physically to open the account.
As a non resident Canadian you file your income tax in Canada not Mexico. You do not have to maintain a Canadian address.