Thanks for this info. So no need for the bill of sale to be notarized? If so that makes things much simpler.I have bought 2 Arimas in US. No big deal. Your list is good. The certificates of title ensure no liens on either trailer or boat. The trailer has to have legible decal with VIN. Check RIV for details of trailer import. https://www.riv.ca/BeforeYouImport.aspx
As you noted you need ICBC binder of insurance before you go with trailer details. The trailer also must be licenced either owner has a current plate on it or you can get temporary licence https://apps.oria.wa.gov/permithandbook/permitdetail/186
You would need to use the Pacific truck crossing border station. Border will collect Taxes on boat.