Just want to clarify something regarding achievements.
If I complete all the missions in co-op on legendary, will I unlock the “I need a hero” and “The legend of 117” achievements?
Similarly if I complete half the missions on legendary in single player and the other half in co-op, do I still unlock those achievements?
Cheers 
Id say yes to you first question but as for your second im not sure
> Just want to clarify something regarding achievements.
>
> If I complete all the missions in co-op on legendary, will I unlock the “I need a hero” and “The legend of 117” achievements?
>
> Similarly if I complete half the missions on legendary in single player and the other half in co-op, do I still unlock those achievements?
First question is a yes, you will receive both of those (I can guarantee that).
Second question is a yes, you will receive both of those (I have a friend that did it that way). The only thing you won’t get is the “lone wolf legend” which is beating the whole thing solo on legendary. Good luck and I hope that clears this up for you.
If you have any more questions regarding achievements I would suggest this site over the achievment hunter site because it can even track everrything for you.
link here