I concur (sadly). The most desirable options are also the least likely (yes, probably completely unlikely).

As you say, the degree of flexibility required to facilitate combined shipping is incompatible with the deliberate constraints of SD - the latter would need to be overhauled to the point where it was a very, very different thing to the SD that we currently know and love.

I think eBay has truly painted itself into a corner on this one, and I anticipate that their 'solution' will be to never get around to allowing combined shipping for private sellers, despite the open day person's claim that eBay is on the case. I'd love to be completely and utterly wrong, obviously.