not totally wrong, I've heard Poles say this (also na weekendzie/łikeńdzie), but not correct either. Also may be misunderstood, na weekend means "for weekend", Like a homework for weekend, ( how dare they give it?) or a weekend trip. I might think you mean what I am preparing for weekend.

no difference. only we say "we" when "w" would be too hard to pronounce.

https://www.duolingo.com/comment/13678518

https://www.duolingo.com/comment/13034812

it's w+accusative. (like with days of the week). I have no idea why.