I can't speak to the other platforms, because I went with Android as the next portable gadget after my N800, myself. But then if you don't like how it runs, why not participate in improving it? The OS is open-source (certainly the portions you're describing). If it wasn't, the hacking community wouldn't be doing the crazy things I've seen. As for supporting their phones, the good news is that Google only makes ONE phone. The rest are supported by a PLETHORA of manufacturers. I can't say the same for Maemo. I hope I can say the same for MeeGo but that's still not something I can walk into a store and buy today to get away from the Nokia nuttiness.
No, you won't be able to. As I said, we're not going to get a gNewSense-like distro soon. But Ubuntu-like (with propietary firmware, 3d drivers, ...). Well, Mer was made by -- guess who -- "the free software community". Not Nokia, not the council, not me.
They're maemo and MeeGo... "Meamo!" sounds like what Zorro would say to catherine zeta jones... after she slaps him for looking at her dirtily...