Friznit, I recommend you reply to the official ECP release thread,
here.I'm sure you'll get an answer here too but since there's a thread for ECP already, no need to have two.
Regarding your questions:
1. EECP is a modification thah replaces the default units, and the same time has the ECP effects and other cool features. A new EECP is underway using the latest ECP
2. Hmm, not sure about this one, never had to do it. There was some talk going on about it over at the official forums though.
3. Not sure since I didn't work on ECP, but AFAIK, there shouldn't be.
4. Not that I know off, but one can easily edit the default island config for it to have ECP stuff. Again, this was discussed at the official forums, I'll give you a link.
5. Excellent!
In the mean time, the all other missions should work rather well with ECP too. Though, it's always nice to see missions that give ECP the full credit it deserves.
And, as for the islands,
here is the ECP thread over at the official's. On that page of the thread there's also some islands configurations. Take a look.