To elaborate on what @jroberts said, I'll also answer the questions.
1.) The PMDG 737, and other FSX aircraft, cannot be transferred, period. The aircraft in FSX and X-Plane 10 rely on a different file layout and a different physics system. They cannot be reliably placed into another simulator. Luckily for you, there have been rumors (a rumor) that PMDG is planning to bring the 737 to the X-Plane 10 platform, but this requires some major reworking. They have already released the Douglas DC-6 Cloudmaster for XP10, however.
On the hand, scenery can be transferred, but this is known to be a somewhat strenuous and perhaps tedious process that requires patience and some background knowledge on file format. But it IS possible.
1a.) Pretty much anything else can't be transferred. Utilities, FSX plugins... they are coded differently than the X-Plane core entails.
2.) It is possible that the X-Plane DEMO, before release, will only be available to developers and other reliable beta testers. However, I am sure (and I think Laminar Research themselves may have said this) that a DEMO will be available once X-Plane 11 has been officially released to allow people to try it out before purchasing.