Let's see going straight down

1) Yes
2) Yes (seems like it from the Ovi Apps require Corporation +.....) thread.
3) Yes
4) You can't install Google Maps unless Google is offering a Google Maps application on the n900 that we aren't aware about. There might be Maemo Mapper in the future. Then there's maemaps a special interface for Google Maps. And then there's just Google Maps' website itself.
5) IF you buy it in the USA your warranty is only valid in the USA. E.g. if you want the device fixed you gotta send it to the states (and then they may not send it to NZ thus resulting in you having to get someone else to act as a middleman in the USA).