"I can't be shorter for you, you'll have to make due and know I won't let you fail." Because he wouldn't. Forced-upon embarrassment does neither of them any favors and that Lucifer is letting Istredd lead as much as he is means the trust is mutual.

It's always hard for Lucifer to give over control in most circumstances.

Formerly? No, not really. There's always a bit of appreciation for the arts spread across them. And a vessel may have some experiences to draw from. For example my last vessel was a rockstar. A bad one Lucifer would say, but that's besides the point. And also not where Lucifer's actual skill comes from; it's all churned together from various vessels and his own dexterous fighting skills blending together in the same sense.