There’s no diff between demo and purchased versions, except the demo has a time limit.
Any files you write with the demo will be safe, never fear.
The academic discount is 12.5%. I did not ask for it, because $40 is a huge value, considering you get free upgrades up to 2.0 and then a big discount on that. But I’m sure you can apply for it, at some point in the ordering process. You might need to supply a bona fide .edu mail address or a campus ID number.
As druid says, there is no differences between the trial and a licensed copy save for the removal of the time restriction, which is by the way very generous. You get 30 days of actual usage (not calendar days), so for many this works out to be quite a bit more than one month. When you purchase a licence, the software will be exactly as you’ve been using it except it won’t nag you any more and won’t expire. You’ll have full access to everything you were working on during the trial, you don’t even have to download another version. The unlock code works right in the demo version, so it is very seamless.
If you do get down to the end of the trial and decide it isn’t for you, you’ll get one grace session in which you can save your work into non-Scrivener formats. Either use the full Export feature, or Compile to produce a manuscript version, or both if you wish.