Re your last question, it's not the 789-44 that drives the receivers; it's the power supply.

I was not able to locate ANY information on this device directly on the Xantech site, but found pdfs on their site by googling the model number. In its instruction sheet, it's said that the 490 draws 10 mA and can output a maximum of 100 mA (at which point, of course, the power supply would have to be able to output 10+100=110 mA). So, yeah, your average Xantech regulated supply should easily be able to run two of these; most of the time Xantech blocks only draw about 5 to 10 mA per emitter LED. Add up what you've got and locate data sheets for the power consumed by any other Xantech devices.

Don't mix brands.

Now, about the green light. First, it's been years since I've looked at a 490, and I though ALL the talkback LEDs were red. But maybe it's nicer to see a green LED on something stuck through the face of a cabinet. However, it should only come on when BOTH of these things happen:
a) one or more emitter LEDs are connected to the system -- no LEDs, no talkback light EVER; and
b) the sensor is picking up IR.

That constant green should not be on. There might be IR coming from something in the room, or some kind of RF interference could be being picked up by the sensor.

About the power supply: be sure that it actually delivers twelve volts. That means it must be a regulated 12 volt supply. Because non-regulated supplies do not deliver the rated voltage, Xantech used to sell 9V supplies and they warned against using 12V supplies -- if those supplies were unregulated. I think people found this confusing so they just started insisting on 12V regulated supplies. If you're not using a Xantech regulated supply, you might be killing your parts.

Yes, when you disconnect the ground lead, the receiver does not work and the LED comes on.

The 480-10D is not working right. Try using just one receiver, then just the other one, to see if they both work by themselves. Verify that your power suppl voltage is actually 12 volts.

Yours is, by the way, an excellent first post. A LOT of people ask totally worthless questions by not providing enough information for a good stab at an answer. Welcome to the site and thanks for asking something that doesn't require asking WTF you're talking about!