I feel like we as a community overthink cover spreads a lot
Sure, sometimes there are connections / hints from covers.
But if i'm being honest, it feels more like Oda put random shit back then and later in the story found something cool in the covers and decided to make a callback. Like most things in the story really. Maybe he just decided to draw something he found interesting in the covers, and because he liked it, he decided to bring it back into the story.
Oda is pretty smart and a great writer, but i feel like sometimes we take his "GODA" thing way too far...
Oda is a master of dropping little random things along the story and picking them up later on without having planned them from the start. Sometimes it doesn't work tho.
But i could be wrong and he has all this shit planned out, it just doesn't feel like it.