1. maemo5 will stop being developed as soon as N9 comes out, based on the previous Maemos.
2. You can install MeeGo now. It doesn't look too good right now, but it could be very good.
3. You can install android now, works really good, no phone yet.
4. don't know.

The release of the N9 will change things overnight, still Maemo but a new UX. But i also think the N8 and E7 will distract a few. The N900 is a year old, and many people would like a new device, many already have a new, Android typically.

I will most probably get a E7 when it is released, unless the N9 has materialized by that time.