If you are travelling sooner than the next 3-5 months, then you could fly to the US and cross the border by private vehicle and CBSA will assess your admissibility at that point. Bring all documents you can to support your status. CBSA can verify your status at the land border.
Unfortunately, there is no way to board a plane to Canada without PRTD or PR card. Thus, you should apply for the verification letter ASAP (#1 above) since you will need this in the future anyway.