1. There will not be a Maemo 6. It has merged into MeeGo. 2. There is no Harmatton. That was the code name for Maemo 6 (see point #1 above). There is no MeeGo/Harmatton for the same reason.
3. There are two builds of MeeGo. An Intel-based build and an ARM-based build. They are the same with regards to functionality. The only difference is the hardware layer (similar to Intel and ARM builds of desktop operation systems).
4. The N900 is being used as a reference device for MeeGo. That means that is it not the same as NitDroid / Mer / or whatever. It means that the people who are building MeeGo are using the N900 to ensure proper functionality. All drivers, programs, etc... for MeeGo 1.1 will function on the N900.
5. Not officially supported by Nokia does NOT mean it will not work. It means at the very least, it will not be available from Nokia's website for download and that it may also be missing the 'hands' animation and Nokia logo. There may also be some Nokia-specific items like themes that will not be included.