Disclaimer: I've only tuned using a Wolf3D V500, so my EMS experience is limited.
I think they are compatible. Not sure if it is an "upgrade" or not. There are slits in the "chopper wheel" and there is an optical sensor that can read them.
The CAS can only tell the ems what position the Crank / Cam angle is. It is up to the EMS to drive the coils. It is up to the user to configure it how they want (wasted spark, sequential, etc). You also need to know if the EMS has some build-in igniters of if they only send a logic signal. If they only send logic, then you need igniters integral to the coil(s) or somehow run separate igniters. You can probably configure it to run using the stock coil / distributor / igniter if you want.
Yes. Not to be harsh, but these are very basic questions that you should know (research) before tackling an EMS install. The "C" in CAS can mean either crank or cam. In the '82-'83 L28ET the CAS is connected to the distributor shaft, and the distributor shaft is geared 1:2 with the crank shaft, it will indicate the cam angle. However, the '81 L28ET has a crank angle sensor.
Not sure, but that "bang-bang" system is hard on your turbos. I know 510Six has set up a nitrous spool-up system for launching, but I don't think it is like the Rally cars have anti-lag where it is also used between shifts.
Hope this helps and read, read, read!! (you're in for a lot of reading, I know first hand, it's not easy the first time installing an EMS ) There is a lot of information in our engine components forum's sub-forums