Why can't Earthland magicians use magic in Edolas without the X-balls?
To answer your first question: because it's said so. (Possible explanation could be that exceeds were laid there so their bodies has some mechanism that they just needed to awaken but mages needed the X-balls for that. But this is just a speculation as we only know as much as it's said: mages needed the X-balls, exceeds didn't.)

To answer your second question: they were not supposed to have the right mindset. They were pretty much in doubt about everything especially about themselves. What are they, what are they for... what they knew so far changed quite drastically in a few hours of time. While it's not stated explicitly in Fairy Tail, in most of the other universes (including RPGs) using magic needs quite some concentration, so that "right mindset" thing can be more crucial than you think.