When Bloodraven disappeared in 252 while ranging as Lord Commander, do you think he genuinely got lost and stumbled across the Children and decided to stay once he realized what was coming/what his powers could accomplish, or did he set out intentionally all along to desert for a higher cause (as in, he was already aware, through lore or sorcery, of what was beyond the Wall and sought the Children out if he discerned the return of the Others)?

I don’t think he planned to desert from the beginning. After all, why would he wait almost a decade and become Lord Commander if that was his intent?

Rather, I think Bloodraven learned something out on a ranging that led him to believe that it was more important for him to stay where he was than come back. 

My hypothesis is that Bloodraven’s ranging was a cover for his implantation in the tree, which he knew would make it impossible for him to return.