My opinion is that with all the phones you mention I could be happy - if I was to be stuck with them that is.

But since you have the present choice of keeping an N900 and you have already been through the worst part of owning an N900, I think you should sit back and enjoy the sunny days.

As for the "external charging of 2nd battery" strategy - It is nice to have an extra spare battery - it allows you to go out in the wild for a weekend without losing your link with civilisation or on 24hr train rides in less developed countries.
Never mind the wear on the back cover. I have the smallest pin broken off it but it still fits nicely. Some particles of silicon oil on the lid fittings may help.