Hopefully, Chip 'n Dale Express will step in and give you the voice of authority

, but I will take a stab at this....
Going to the Poly, you can probably take one of the boats that travel between the WL, FW and CR and from CR, take the monorail. That is a fun way to go and a choice DH & I have made previously. I think it is still running at that hour, but not positive.
Going back after 9:30 could be trickier. You'll need to see what is running when for that date, but in your shoes, I think I'd consider a taxi as a back-up plan. There have been a few times when taking a couple of legs to get from one resort to another was fun going, but a headache getting back. Much will depend on your energy level at the end of the day.
DH & I have not tended to rent cars when flying down, since the WDW transportation is generally very good. We just keep a little Taxi
budget so we won't ruin a wonderful day by getting frustrated over slow transportation late in the evening.
HTH!