You will not regret it! We stayed off-site once and we said never again. Occasionally we will for a few days before moving onsite, but never totally offsite, it's just not the same! 
As you have made this statement as fact, can I ask what your source is?
If Disney was having trouble filling up their rooms they would be offering more discounts than they are. They would also not have spent money on adding rooms as they have. I don't doubt some people who used to stay on site are now staying off, but I bet the opposite is true too.
Not if you valet at CR like we did. Leave park, walk to car be home in 15 minutes. I don't know how long it takes from TTC we did valet both days we were at MK.
I see OP has picked the Poly...I think you will have an amazing vacation. We had a lot of fun when we were there...even though we didn't sleep there. It is a beautiful resort
You valet at a resort you are not staying at? Shame on you!
http://storytimeorlandorental.com/
Here you go!!! If you book I hope you enjoy it as much as we did. The only real complaint I could make is that the carpet was worn in couple of places, but otherwise the place was super clean(and I am very anal about being clean) and you are immersed in WDW decorations. The owner was great to work with and responds very quickly. The rest of the resort was very quiet and well kept. Even a great park for the kids to play at. They also do have transportation to the parks, but we never used it so cannot comment on that.
Wanted to add we stayed at the Storytime Kingdom
The OP is traveling in September too. I stayed in a 4-BR house 10 minutes away from WDW for a whole week for a little more that the OP is spending on 2 nights at the Polynesian. If I didn't own at DVC I probably wouldn't be able to afford to stay on-site anymore. $422 per night for the Polynesian?!? The prices are crazy expensive!
I have similar thoughts sometime! Sometimes I want a little luxury at a value resort price. Sometimes I just do exactly that and stay offsite.
I might look into Hilton Bonnet Creek - it is practically on site (provides its' own transport shuttle to the parks) - very nice, lazy river pool. The only downside are the extra charges - resort fees, taxes, parking - however if you know all the costs going into it - it is easier to make a decision. The last time I was looking Hilton Bonnet Creek came out to $20 more than Pop would have been for the same time. The Ritz, Waldorf Astoria, JW Marriott GL all offer deluxe experiences for a moderate/low end deluxe price.
So you don't have to factor in a "car" you just have to factor in roundtrip to the airport. Over the course of my mythical 4 night stay you saved $260. I think the resort fee is around $20 a day so $80. So if you can get too and from the airport for less then $180 you come out ahead. (Plus you get a better bed and a real rewards program.)
I have stayed at the Poly and at the Swolphin and the bus situation is just about the same between the two. The Poly shares with the other MK resorts and the Swolphin shares with the other Epcot resorts. I agree they are not plentiful and super fast like the Value resorts, but they are perfectly adequate. There is really no need for a car. We stay at BWV often and usually leave our car parked in the lot for most of the trip. You can take the boat or walk to Epcot and DHS and no one in their right mind would drive to the MK parking lot and take a tram to the monorail/boat when the buses drop right outside the gate. The only park where I might consider driving v/s taking a bus is AK and frankly it's not a big deal to take the bus there either.
