Im pretty sure Rock D. Xebec was who spread the information about the 4 road poneglyphs to reach Laughtale. Roger, Mom, Blackbeard, Whitebeard and Kaido heard that island exist from him.
Well, Roger was the one who discovered the existence of the final island when he got to Loadstar, and that the Poneglyphs were the key to get there, and that happend before the God Valley incident.
WB didnt know about it until Roger told him more than 10y after Rocks have been defeated, and most likely he was the second guy in the Rocks crew.
Seems much more like Rocks was after the true history behind in the Poneglyphs to overtake the WG and take Imu place as King of the World, thats why the attacks on the Celestial Dragona. Very likely Kaido heard about Joy Boy from Rocks, and i wouldnt find strange if BM Road Poneglyph was originally from Rocks too, but there's no indication he was after Laftel or that he even knew It existed, what Sengoku a indicared that Rocks pehaps even realized about the existence of Imu, and wanted to take his place.