I've been using Search & google & I can't seem to find
info on this.  Can I play the game on Legendary & choose
any mission in any order & still get the Achievement if I've
completed all missions on Legendary?  I don't particularly 
care to play the full game on Legendary sequentially since 
it seemed so long & tedious on heroic.  I was thinking of 
starting by finishing the last mission first then moving 
backwards, basically to get the harder/frustrating parts 
out of the way at the start (or at least see if I can), that 
way I won't commit tons of time into it just to walk away
at the end.

As far as I know, yes.

Yes you can play in any order, they check by the mission so as long as they are completed it counts. Also if you are not trying to get the solo on legenday achievement then co-op levels you have completed also count.

Excellent. Thanks guys!

God I hope so I only have one more mission to go finally.