Before answering these questions, I just have to say I haven't tried using any of the configurations you've mentioned, but would be fairly confident it would work. We're currently using Oracle database 10.2.0.4, but I have both Oracle 10 and Oracle 11 clients on my machine, and I can use either version of the client and still get the same results for the things we do.
For question number 1, yes, but only because you use the word "should".
For question number 2, yes the 11 client has new features, but you could probably still get by with the 10 client.
For question number 3, probably...
Nothing can substitute for your own testing, since you know best what you and your company do with this software. You can probably find documentation from Oracle about new features for each release, etc, which would be worth your while to look into.