Lance,
Today the EMS software supports displaying the EFIS on the screen. It has since the day we shipped the first EMS-D10, and the EMS-D120 also supports it, including split screen modes.
The reverse (EMS on EFIS) is coming out soon. We are just doing our final flight testing on this software and hope to have it out in about a week.
So the answer to question #1 is yes, it can do it today.
For #2, we do support some of the GRT sensors. The EGT and CHT for sure, and I think the rest of the sensors that GRT uses are the ones that come on the Jabiru engines. We support the Jabiru sensors so you should be fine. We'd have to discuss any sensors that are not EGT/CHT or didn't come on the engine from the factory.
For #3, the upcoming software update will let you use the OAT that is on the EMS for the EFIS TAS and DA calculations. You can't do it today, but you will be able to soon with a free, in-the-field update.
And just for future reference, the 7" EMS is a EMS-D120, not a D100. Don't want you to end up ordering an EFIS-D100 on accident!