I have some questions:
1. Can the client be used with the rear TV? Will it be controlled independently from the front tv? How do I install the Genie and the client?
Yes, you will need to run one coax cable from the antenna to the Genie and another to the Client. The SWM head will only have one coax output so you need to add the SWM splitter in your setup somewhere either inside or outside depending upon where it makes the most sense to run the 2nd cable.
2. Assuming the new HD setup works, what will be needed to return to the dome (SD), an old SD receiver(which I have), and everything work just as it can from the factory (if I had trouble with new system or someone stole the dish)?
Any DirecTV receiver will work that does not require SWM. Currently that's only the Genie (HR34 or 44) and the H25. Or you could add SWM8 hardware to your dome as well and use the Genie and clients with that setup. You just won't get the HD versions of your programming.