I assume to check in at the Poly we can stop near the main entrance and offload luggage, then park closeby to check in and then move the car to a permanent spot. In fact doesn't bell services meet you at the curb? I seem to remember that from all of my other deluxe/dvc stays.... So I shouldn't have to wait on more than one line (unless something has changed) then go in, check in physically and then head to Epcot. if I leave at 8:30 I should have time for all of that I'd think. I was going to walk to TTC just because the poly parking lot is sort of more toward that side of the resort property anyway. I don't THINK I am setting myself up for stress that way!
I had originally planned to walk to Garden Grill for breakfast then stay til we are done and go back to BWV and move the car to poly and then figure out where to go from there but it seems like a LOT of extra walking since I would be coming in from the international gateway... I try to avoid extra walking when we're walking so much already. But then I started thinking maybe I should just move my car to the poly before breakfast when things are quieter in the 'world and then just not worry about my car in the middle of the day and have the freedom to monorail wherever.
You have a busy day planned! I guess some would say it's counterintuitive to check out of an Epcot resort, check in to a MK resort, and then head straight back to Epcot to eat, but Poly's location and quick access to the TTC makes that pretty doable. I am a fan of GG so I'd probably do the same thing if that's the day I had pegged to eat there. I'm sure you have your reasons for the late breakfast, but any way you'd want to do it earlier? Like PPO?
To answer your question, yes, what you describe should be pretty easy. It's been a few years since we've actually driven and self-parked so I don't exactly remember if they will let you park off to the side for a few minutes while you go check in, but they probably will or you can drop your bags with bell services then self-park before you go check in. You can also valet but it's a pretty steep daily charge (like $20+ I think). We had a rental for our July trip and I have to admit I did do valet because I just didn't want to mess with self-parking and wanted to get on with our day, but it was overkill.
If you'll forgive me while I map out the times (mainly just want to make sure this works).... this is roughly what it looks like I guess.
8:30 - Leave BWV
8:30-9 - Drive to Poly, Drop Bags, Park
9-9:30 - Walk to TTC, Monorail to Epcot
9:30-10 - Security, tapstiles, walk to GG
To me, this all looks like you'll be good. Not a ton of room for error (mainly concerned with something like a big monorail delay), but there's enough fudge time in there I 'think' you should be fine. If you want to be very comfortable maybe leave more on the 8 side of things than 8:30.