Canadian ex-employee now doing contract work for US employer

[SniffingGeneral]

First, my situation:

I used to work for a company in the USA (where I lived and payed taxes for 4 years on an TN visa, and then an H1-B visa, I did not file in Canada during this time). Last year (in October) I moved back to Canada (Orillia, Ontario), and started working under contract with the same company. My employment with the company was terminated at this time. When I left the US I filed a 1040-C (U.S. Departing Alien Income Tax Return) and paid tax on my income for the period while I was still employed at the company.

Shortly after arrival in Canada, I moved into an apartment and signed a contract to do some work for the same company (working by myself from home). When I started my contract work, the company had me file a W-8BEN (Certificate of Foreign Status of Beneficial Owner for United States Tax Witholding) under article VII of the tax treaty to avoid doing any US withholdings (since I no longer have any US establishment). The amount of my contract for the remainder of 2011 was around $15,000 (USD).

In the new year, the company and I agreed to extend the contract for an additional 3 months and increase the total amount to approx. $30,000 (USD).

Now, it looks like the company wants to extend another contract for additional work for another 3-month period.

My questions are:
1) I understand I need to file a 1040-NR in the US with the W2 provided by my employer for the period where I was an actual employee of theirs. Is there anything special I need to do to inform the IRS of the W-8BEN at that time? I assume I also need to file a tax return with the state (NY) as well?

2) Do I need to file a tax return in Canada for a) the entire year (including the portion of the year where I was employed directly by the company and paid taxes in the US)? or b) the period from the end of my employment (and when I moved back to Canada)? How do I file if I don't have a T-4? That is, how do I report my income derived as a contractor?

3) What should I do for the coming year to ensure I am in compliance with canadian tax law? Should I have been charging HST on the invoices that I submitted to my client (my previous employer) even though they're in the US? Do I need to make additional arrangements to pay a portion of my tax throughout the year since the company is not doing any withholdings?

[JGCA]

You file 1040NR and NY NR return for the US source income you earned while you were in US. No other notification to IRS is needed.

You file a CND T1 return from day you returned to Canada do not report income your earned while in the US on your T1. Your sub contracting income is reported as business income where you report the gross income less any business expenses claimed.

You are now resident of Canada you have to register for GST HST or TVQ if applicable when your worldwide sales reach $ 30K or more in the year. Since you arrived in 2011 the sales in 2011 were not over $30K so 2011 is fine for 2012 once you hit $ 30K CND you have to register for HST ( ont 13%) to be charged on the sales to your CND clients but NOT US these are Export zero rated unless the client has a PE or offices in Canada.

You do not have to make any prepayments unless CRA asks for installments since it will be your first year wait for them to inform you if they require you to make quarterly installments for 2012 if not as long as you pay the full tax owing by April 30th no interest will apply.

For HST you can elect to file quarterly or Annually also, annually would be less work.

[SniffingGeneral]

Thanks! One final note of clarification:

Currently, my ONLY clients are in the US. Since I don't have to charge them HST (no offices or PE in Canada), do I still need to register for HST?

[JGCA]

Yes the obligation to register is based on sales volume worldwide not tax charged once you hit $30K you have to register