But ... that would mean that I still have to get my hands on an HR10-250 ... right? If I went with whatever Directv is offering at this point, I would have to upgrade the dish, wouldn't I?
Yes HR10-250.
If you contact Directv, they will want you on the R15 the SD DVR or the HR20 the HD DVR. Those will work with a 3 LNB dish and you can couple the OTA and D* signal. However, if you go with an HR20 HD DVR, they might require you to sign up with the HD package. However, if you contact retention group, you might get free HR20 and free HD package for 6 to 12 months. Now you can do a D* single wire called Single Wire Multi-Switch that allows 2 D* signals to come down on one wire. Here's the other catch. I dont think you can couple OTA with it though.
So the skinny is, your 1 wire is very limiting and its going to really reduce your options. Also not wanting to pay HD access fees and DVR fees also limits your options.