Hard to pick out the questions but an update and I'll try.
The Nexus 7 arrived and a Transformer with the keyboard is in shipping.
I can't tell you your choice. Here I can get by just fine with the units because I have other options. We can also put files on a drive on the local network. -> What I do encounter are folk that want to carry their life's collection of MP3 and video with them. For those folk, they'll have to get what they want. There is no discussion with them.
KF 7, 8.9 "Proprietary"?
If you research this you find there are ways to side load apps. But I fear this is another area where if you want Android THE OS and nothing less, you won't buy this. However it's a great unit, plays 1080p in the big screen far better than any other device I've seen to date. -> Bottom line. Depends on what you want.
Amazon Prime other features?
There's a lot of video content and more that you get for your Prime account. Free 2 day shipping and well, I won't go further on that. It's nice to have all of many old shows just a few taps away. -> The KF HD 7 is now part of the home HD system.
Hope this helps.