Tricky to say, but maybe he did that to steal the gum gum fruit ?
shanks got that injury from freeing fisher tiger first
No he got it from Blackbeard. He specifically tells Whitebeard he got the scar from Teach when he asks him to stop Ace from fighting him
As it shows Roger being executed in this chapter I assume it is after Roger gets executed but before he forms his own crew.
if you are right about him, being a double agent it would explain why he took the gum gum fruit to maybe get/claim a spot with the saint elders since that was his goal while meeting Harald. Since, Imu wanted the gum gum fruit to be retrieved. Something also tells me that the twins could had switched places (like i said i could be wrong. shanks acts more like his brother when it comes to the serious things) idk i could be wrong but it's a theory.
Hold on, when shanks join knights of god, was it before he creat his own crew or after and became a double agent. Since his eye was cover in bandage it must've been what Teach did.