In terms of crossing the border the temporary plate is okay as long as you have adequate insurance on the trailer. You may have to insure the Border guard that the trailer is not being imported into Canada.
If a dealer takes more than 90 days to register at the DMV a lot of red flags start going up. Your dealer maybe in the middle of bankruptcy negotiations which is why nothing is registered as sold so they do not lose the money they made on your sale.
Just a reminder you are entering a different country with different rules and regulations. It doesn't hurt to call the border patrol and just ask they will give you the answers to all the paperwork Etc that you need.
Ownership and Insurance liability are the key issues.
I hope you can resolve your issues good luck.
