Am I eligible on the transit visa?
 

I am not on the visa waiver list and am in the US with a valid US visa, and plan to catch a flight from Vancouver to Hong Kong. I plan to enter the border from Seattle on land, and then go directly to YVR. Am I eligible to apply for a transit visa or I have to apply for visitor visa?

The above is scenario A, for another scenario, that I fly from NY to YVR and want to do a stopover in Seattle (enter by land too) and then go back to YVR on land after several days, am I eligible for the transit visa?

Your eligibility would depend on the US visa type and passport you hold.
If you have a green card you are admitted to Canada holding any nationality passport.

I m not on the visa waiver list, so I have rely on the transit visa if that allows me to enter on land to transit through air.

I don't think transit visa applies for your particular situation. You have to be able to enter Canada regardless of onward travel plans.

Yes, because once you cross the land border you could go anywhere.

We know you're going to the airport, but the border officers can't guarantee it.

Unless you are driving an airplane over the border the answer is NO.

Only nationals from approved countries who meet a number of conditions qualify as a TWOV passenger:

You are disqualified as it appears that you will not arrive in Canada from the United States on a direct, non-stop flight on an approved airline and you will not be remaining in an isolated transit area while waiting for a connecting flight.

You wil need to meet the normal conditions for entry to Canada

If you only have a turist visa B1/B2 you have to apply for a transit visa in the Canadian consulate, better if is in your home country before to arrive to USA.

To apply for a transit visa to Canada is free, you do not have pay any fee, so once you get the transit visa you can not go out of the airport, just wait for your next flight.

If your VISA from USA let you live and Work in USA and you have your green card, then you do not need transit VISA for Canada, with your Green Card and your Passport you can enter to Canada with no problem.