it seems to me shanks got the mark after roger died we hear of him in this flash back that he met harald mary geoise so shanks probably got his mark there
It didn’t seem that they cared Roger according to the summary ordered his men to go for the treasure not helping the civilians
and we saw the Rocks pirates majority of them after Shakky the rest after devil fruits and treasure
Rocks himself main objective is getting his family and leaving
He seem to be a foot soldier who joined very recently this is the first time he witnessing the tournament and I doubt most people in the marines or the world know of it