Hmm..
so, I called Aeroplan again.
The dates are
8/25 - NYC-CPH
9/2 - TRD-ARN
9/5 - ARN - EWR
I was told that if I don't show up for the TRD-ARN segment, the ARN-EWR will NOT get forfeited because the dates are different. I was told to call SAS to confirm.
So, do I go with what "Sam from Montreal" told me?
My other option would be to have them cancel the TRD-ARN segment - they were willing to do it for $90 pp without changing the miles. They weren't willing to change the departure airport without extra miles though.
I will still need to buy MOL-ARN segment then, roughly $130....$700 to save 3 hrs of drive time... we'll see.