I would say your first error when you modified the URL was the /%2f between com and ca which is equivalent to // instead of the required single slash. In your sample in post #4, you removed the & but not the fc before the showpointprice item.
I'm assuming the 3583 matches your current points total.
I generated a search with the min price set to 2400 (as far as I am aware, that is the minimum points price) and the max price set to null.
https://www.kobo.com/ca/en/search?qu...cmaxprice=null
To use your example modified to clean the errors:
https://www.kobo.com/ca/en/search?qu...cmaxprice=3583
I do get Kobo Plus books mixed in as well since some have a points price as well.