contract work for us employer - in canada and us

Hello,

I'm a biologist living in Canada. I have been offered some work - roughly 4 months of work over the next 2 years - for an employer in the USA. Most of the work will be data analysis which I will do at home in Canada. There will be a small amount of fieldwork which will be done in the US - approx 2 weeks/year.

I would like to accept this offer of work, but would like to figure out what needs to be done to make sure it is all legal.

Will I require some sort of visa to perform the fieldwork in the US? - a TN perhaps - and would this cover the entire 2 year period, or would I need one for each trip?
Will the US employer require some sort of taxpayer number for me? - I do have a SSN as I have actually worked physically in the US in the past.
Will I have to file both US and Canadian taxes? - the US amount will be relatively trivial - ~$10K/year - Of course I will include this on my Canadian tax return.
I expect I will have to set myself up as self-employed in Canada, and will have to pay into CPP, EI myself as the US employer won't bet set up to do this.

Any advice is very much appreciated!

Few thoughts and questions on somewhat similar situation:

There is B1 visa that can be used for activities such as business meetings in US, but you are not supposed to be paid by sources in US for the activities performed during the B1 trip. If you are employed by company in Canada, B1 would be fine, but for Self-employed, it is an awkward situation.

I develop software for US clients and like to visit sometimes to provide training, the training would be a part of project which I will eventually bill the client. In such situation can I enter using B1 visa? and what am I supposed to tell the Immigration if they ask me "am I getting paid for the business trip?" Setting up company in Canada would be big hassle just for rare short visits to the US. Anybody has any opinions? Thanks.

If you have a Cdn business, and have the US pay your business, B1 is fine. Otherwise, entry on TN is also acceptable, if you have a degree. The contracts would be between US and Cdn firms.

Since this is contract work, and you are not an employee, you will have no tax issues in US, but you can still get TN if needed.