My line of thinking is that Pluton would potentially give Blackbeard a severe advantage given its supposed Island Destroying capabilities. Maybe it'll require something else to function, but that still feels like too much power on BB's end.
With Shirohoshi, her being unwilling to assist BB would be the point. He has the weapon, but we'd know for sure he'd be unable to make full use of her, at least, not immediately. Not only that, but she's invading Fishman Island seems like it'd be more up Blackbeard's alley as it's an easier target than Wano.
I also don't think Wano's borders could be destroyed by Blackbeard's ability, personally. Again, it's implied Zunesha is required for that and I doubt Oda would set up that plot point and not follow through, especially since it relates to Joyboy and the Void Century. I also don't think Oda would have Momo and Yamato fail at protecting Wano after he made a bid deal about "passing the torch" to them.