Thank you all for your responses. Here is what I did:
First, I called WDW and spoke to a wonderful woman who then called Guest Services and the resort and was able to confirm that we could use
DCL transportation from the resort to PC and that leaving the car at the resort during the cruise would not be a problem, but from WDW’s side they were uncertain that DCL would allow us to get transport from PC back to the resort without a resort reservation. She mentioned that they did not know this and didn’t know much about DCL’s system of who is and isn’t allowed to use the transportation because there is virtually no communication between the two. So according to this info, the solution would be to make a reservation for the day of disembarkation. I then called DCL to see if that was our only option, but the helpful woman at DCL told me that it is not necessary to have reservations at a resort to book the PC to Resort transportation, but that she was uncertain about being able to keep the car parked at the resort during the duration of the cruise but she didn’t really know and had never been asked about it before. So then it seemed that we could go ahead and book transportation, but I wanted to call WDW again just to reconfirm. I called WDW and explained the whole thing to another helpful woman who called Guest Services and the resort again. She said that leaving the car at the resort should not be a problem and that they would just have me fill out a form about why the car would be there and that it posed no danger.
To address the concern of cost, yes, I realize that driving is likely cheaper (I only say likely because I haven’t crunched the numbers myself, not to undermine anyone, please don’t take offense). The reason I would like to take the DCL transportation is for the insurance. We are checking out of our resort the morning of the day of our cruise. I know it is only an hour to an hour and a half drive, but a lot can happen between here and there. We usually drive/fly in the day before, but not this time. I am wanting to take the transportation just for the insurance that the ship is far less likely to leave a DCL sponsored transport than two women in a Prius.
We are still considering our options at this time.