1) I'm sure 4GB RAM will do, but in case we want to use it for more in the future, is a 6GB MBA even available?
4GB is sufficient for general usage like document editing, internet browsing, and some light photo and video editing. I have MBA with 4GB RAM and so far it works fine.
6GB is not available tho, there is 8GB and you need to order it from Apple Store.
One more thing, the RAM on MBA is non-upgradeable, so think carefully before you choose.
2) How is the screen after long use (being non-Retina and only 11")? Sometimes she is up typing and researching until midnight or later. Would the retina be much easier on the eyes?
Depends on people, but I would say that it feels "claustrophobic" due to its screen size.
3) best place to buy it? (Best Buy, Apple store). I think Best Buy still has interest-free financing for 18 months. That sounds appealing right now as Christmas is around the corner, we are paying for her grad. school, etc. However, I believe Apple has no restocking fee whereas BB does.
Apple Store.
4) We have not used Apple OS on a computer. We have iphones and an ipad. Will the adjustment take very long from Windows to Apple's OS?
Nope, OS X is very easy to use. You will get used to it in a day or two, or maybe less.
5) any other important factors you all can think of? Does the 11" MBA sound fitting?
11" is perfect for travelling. If you want to use it more like a desktop, then get the 13", or you can get the 11" and connect it to an external display when you need a larger screen.