Just to follow up, I did resolve my install problem last night, by virtue of having my call taken by a DirecTV supervisor last night. That's some random luck for ya. An installer was here today and he replaced my round dish with an 18" elliptical, and ran the third line for me. So it's a pretty happy ending to a long ordeal of hold times and "pass the buck" customer service.
Maybe you could get back some of the extra $100 you had to shell out.
Maybe... But at this point I'm using one of the receivers they gave me for my third line (not for ReplayTV use, just as a standalone tuner) so it'd just be the one receiver I'd get credit for.. And then I'd have to ship it back to them... At this point I think I'll just keep it around in case one of them goes haywire or something rather than try to beg for ~$30 net gain.
FWIW, the two ReplayTV-incompatible D10-300s are both Philips manufactured, DirecTV badged.