Good catch on the "mientras" vs. "mientras que" -- I didn't even notice that.
I still say that "mientras" requires the imperfect, though. What I'd learned was that when "mientras" is joining two clauses, you can combine either imperfect+imperfect or preterite+imperfect but never two preterites.
Here's a page that backs up what I'm saying:
http://www.cliffsnotes.com/foreign-l...rfect-formulas
By the way, I just asked my wife (who's Brazilian -- native language Portuguese) and she said that while native speakers do sometimes use two preterites, she's pretty sure that it isn't grammatically correct.