2 questions
As I understand legendaries you need to craft 2 gifts for it, which the girl next to the mystic forge sells the recipes for. These recipes require craft level 400 to learn the recipe, so yes you need to be at 400 in that particular craft.
Regarding MF and chests having used countless MF food buffs and worn several items gaining MF when running the jumping puzzles in Lions Arch, I can safely say that I haven’t seen any difference in loot (I might be extremely unlucky though). As I understand it, what MF does is it provides enemies with a higher chance to drop loot one tier higher than standard for their level, and that’s it. This means your chance of getting tier 6 crafting mats will be higher from e.g. enemies in Cursd Shore the more MF you have.
Gifts are Account Bound so yes your crafting has to maxed .
As for Magic Find no chests are not affected since chests aren’t drops .
This is from the Wiki
“Magic find is an attribute that increases a player’s chance to receive rare loot from drops”