I'm going to disagree with most of your conclusions
First off, as you yourself mentioned an Apple 7" tablet is still a rumor, albeit a seemingly well-backed rumor. As such, I really don't believe Apple is headed in that direction, but if they are then Apple could well take the ground right out from under the Nexus 7's boots. Personally, I don't see a need for a 7" device as anything more than a media reader since the display, to me, is too small to perform well in any real productive manner. At 7", it would live up to the naysayers' expectations of what the iPad would be at its first announcement.
Apple isn't alone when it comes to talking about Android's fragmentation; nearly every tech blogger online has acknowledged that while Android as a platform is apparently superior to iOS itself, no one iteration of Android has really become dominant and Ice Cream Sandwich, despite it's obvious superiority over previous versions, has only managed to wrangle a 7% adoption since its release a year ago. This isn't a fault of Android itself, but rather due to OEM fragmentation simply not pushing the new version onto older devices in an attempt to sell more new devices. This mindset wreaks havoc on customer satisfaction and customer retention. The Google Nexus 7, as a direct Google device, has the advantage of being the first and maybe the only mini-tablet that will have a direct line to OS updates rather than relying on OEM/Carrier procrastination. The fact that the Nexus 7 is $200 cheaper than the 10" iPad gives it an economic advantage though as yet I'm not certain about the reliability of the Nexus giving it a practical advantage.
Finally, I do believe Microsoft may have a viable product on its hands with the Surface tablets--more specifically the ARM version though the Intel version will have separate advantages over the short term. This is more due to the advantages of Windows compatibility more than the Metro interface itself, since Metro app files will seamlessly synch with desktop versions of those apps. It's greatest advantage then, even over Android at this point, will be the integration with the desktop in much the same way Apple's productivity iOS apps integrate with their OS X apps. By removing certain manual steps in the collaboration between devices, Surface becomes the easier-to-use non-Apple solution which also has a much larger infrastructure supporting it.
