Some Questions regarding Coop and Golden keys

Hello Fellow Vaulthunters.

I have some questions regarding goldens keys in coop mode, i tried to google the awnsers but couldnt find anything that

Im going to coop with my friend for the first time tonight, this is his first playthrough so he doenst have guns, keys and stuff, I Have alot of golden keys to spare. So my questions are:
- When we get to sanctuary, is it possible to use my keys to open the chest? So he can get some good guns? Or do i need to use some game features to achieve it?
- Can you send eachother golden keys through mail?
- What did you do while coop? which mode did you use for the best results?

IF there is an other Post explaining these questions please share, i couldnt find it through searchfunctions/
Thanks for the questions in Advance and have a great day! LIEBERMANN KNOWS