WG is and always was about hiding information even from their own people or going behind their back. Akainu points out post dressrosa how CDs went behind their back and did whatever they wanted. I doubt the he actually knows about Imu, in fact, I doubt any other CDs knows outside from rumors. More than likely this was another case of hiding information and only now did Imu tell them that the rumored legend was true and what exactly it was.
Their conversation clearly shows that they just learnt the truth themselves and discussing it on screen to inform the audience. Why didn't Imu tell them before? Well, we'll have to wait and see. My guess it have something to do with the giant Strawhat.
BUT your second statement isn't true at all. Becoming pirate king and finding the OP has nothing to do with being Joyboy. WB could've been a Pirate King if he bothered but he didn't. Roger became one and found the OP. Joyboy is connected the ancient kingdom and after finding the OP, not the act of finding itself.
Also, it's still a rubber fruit without awakening it. It was a rubber fruit when Luffy used it. It showed no other ability in the past until awakening which is just taking Luffy's freedom with his body to the next level. How is a powerup that happened now takes from his previous victories? Did AdvCoC take from Luffy defeating Arlong somehow?
Still doesn't address the problem at all. You don't need to tell the Navy: "This boy is the special criminal Nika Joyboy." The WG can always just lie to them and use any other excuse. "That is the son of the Worst Criminal, we want to make an example of him," "That boy is trying to read poneglyphs which is why he has teamed up with Nico Robin," "We don't like the way he looks", etc. They've lied to the world for much less.
Imu would be a colossal moron of the highest order to ask the Gorosei to hunt down the secret fruit without telling them what the name was changed to. That's just dumb.
Like I said, their conversation is part of the problem. Why wouldn't the Gorosei know the basic info to do their job? How have they been hunting it for 800 years if that were case? Why was it being guarded by the special agents if they didn't know? It's a mess because there's no way to untie the knot without someone at some level looking incredibly stupid.
Whitebeard could have become Pirate King, but every indication is now that the One Piece would have been useless to him and he wouldn't be able to use it overthrow the WG. That's my point. The race for the One Piece now seems foolish because even if you found it, without Joyboy's special fruit you would be out of luck. The issue isn't finding the One Piece but actually using it in any meaningful way.
As Kaido said: "I'm the only one who can change the world!" That is apparently the essence of becoming Joyboy. Now it looks like he failed because he ate the wrong produce.
Also, Oda is now indicating that it wasn't really just rubber even before awakening. Kaido said "rubber shouldn't do that," so Oda is changing the fundamentals of it.
Changing it now doesn't change the struggles Luffy had, but it does change the optics of the encounters. It's no longer someone rising to surpass those with superior abilities with the mundane and generic, but one who had special abilities all along that just couldn't access them for seemingly arbitrary restrictions.
Rubber band overcoming a leopard is just a much better look than Sun God Nika with a fraction of his abilities overcoming a leopard.