I have the same problem.

I dont think

• needs to be first or that all the columns need to be there, I just dont think its possible from the way its coded.  I really hope a dev can come in and clarify.

I am trying to insert new categories to already existing products, I get the same error "use insert, all products are keys"

SOLUTION:

add an updatable parameter. 

I added price to my update csv, and tada!

HOPE THIS HELPS.

Ah this is what I was looking for! Appreciate you sharing your solution.