Why wouldn't he? Dude even knows Vivi and Shirahoshi and the threat they posses. Ain't no way he wouldn't know Luffy and his fruit, especially given the striking resemblance b/w his strawhat and the one we saw at Im's place.
Because Shirahoshi is the mermaid princess. Robin found out immediately after reading the poneglyph her significance.
Imu would need to be told that Luffy had the gum gum fruit unless you think he’s omniscient. But again, who’s reporting that Luffy had that one specific fruit?
The world was in enough danger:
1. They were likely watching shanks and his crew to see if any of them had the fruit
2. They just found out about the son of Roger
Luffy has been hiding behind red herrings his entire life. Red Haired Shanks the last known possessor of this fruit and Ace the son of the previous pirate king.
The WG wouldn’t keep close notice of Luffy until Dressrosa because they had more worrisome things to worry about