I agree with 1-800's interpretation.
Reading through the long form provides some clarity. As I sift through the long form myself, it seems to me:
June 28 2016 was the last date someone could sell their eligible car and still be a class member as an eligible owner.
If you bought your eligible car after September 18 2015, your restitution is 50% plus a proportionate amount, etc. etc. June 28, 2016 demarks nothing, except it was the last day to remain an eligible seller, and the last day a VAG dealer could have sold the vehicle and still have its buyer eligible for the 50%+ restitution - IOTW, June 28 2016 is only relevant to defining eligible sellers and eligible vehicles.
There doesn't seem to be any scenario in which a post-9/18/15 buyer will receive 100% of restitution, even in the extremely unlikely event that all eligible sellers fail to register for their compensation. I say this because even a small (?) number of these used cars were owned by VAG dealers on 9/18/15, and are thus excluded as eligible sellers.
Think like a corporate entity that is trying to limit their losses. There is no eligible seller for those cars, but there is an eligible buyer, just as the OP originally surmised. All that means is that VW is not on the hook for the seller portion of the restitution on that population of affected cars. I can't believe they would chuck those funds back into the seller pool, nor would I blame them, really. I would surmise (pure speculation), that VW will settle up with their dealers through an internal program, just like the dealer buy-back program they put into place early in the scandal.
Common sense also tells me that eligible sellers who register by the deadline won't have to wait for the buyers of their vehicles to get their vehicles bought back/fixed to get their share of the $$$. To simplify things, I'd expect they'll get their payments timely after the registration period ends, just straight up 50% of column 2 on the 1A attachment, no mileage adjustment. So the idea of leveraging the eligible seller (and yeah, privacy laws would probably prevent VAG from disclosing who that would be, anyway) in any way to the eligible owner's payment seems unlikely. Again, tho, that's my common sense guess, and you should read through the docs yourselves. TBH, I already have a good idea of what the settlement means to me as the original owner and am mostly okay with it, so I'm just reading through the docs out of a perverse curiousity.