Delaying him was the whole point once he realized he couldnt kill him.

it was to delay him long enough for a new hokage to be appointed and that they would defend the village against the next attack, he even hints at this before he summons the god of sealing death.

His death was NOT in vain, it gave them time to track down Tsunade didnt it?

Hayates cold is not a plot twist in any way, its just a characteristic of him.
just like Sigures scar (guy who gets killed by Gaara)

they just have them ok? is it that hard to believe that you cannot learn everything about everyone?
you might encounter someone outside with a huge bruise mark and never know why they have it and when it happened, does it really matter how they got it if they're such a small part of your life and you never talk to them?
none of the gennins ever talked to Hayate.