If a non-resident seller’s inventory is stored in Canada (like in FBA warehouse) then the seller is required to register for GST/HST.

Canada Revenue Agency (CRA) registers the businesses for GST/HST.

Based on Canadian income tax law if a non-resident business has a Permanent Establishment in Canada then its Canadian sourced income is subject to tax. However, based on tax treaties between Canada and different countries just storing its inventory in Canada does not mean having a Permanent Establishment and therefore, business income will not be subject to tax. What it effectively means is that non-resident business is required to file an income tax return and claim treaty-based exemption by filing relevant additional schedules.

A non-resident Amazon seller in Canada is not required to get an Import Account but it is certainly beneficial to get an Import Account. By having an Import Account a non-resident business can be Importer of Record while importing its inventory in Canada. It will also be easier to claim back any GST/HST paid at the time of import.

Canada Revenue Agency (CRA) issues Import Account.

No, non-resident Amazon sellers in Canada are not required to incorporate their business in Canada.

If non-resident Amazon seller's inventory is being stored in British Columbia (BC) then they are required to register for PST in BC. Otherwise, Quebec (QC), British Columbia (BC), Manitoba (MB) and Saskatchewan (SK) have different revenue thresholds for registration requirements.