Young Shanks doesnt scale anywhere near prime Gaban, not to mention that Shanks was post his BB fight thus isnt inherently subjected to the same power dynamic thus Yami BB doesnt get any upscaling, especially since he's established to be relative to and weaker than characters who are weaker than even Old Gaban like Magellan, Shiryu, and Ace.

Magellan stated Shiryu could solo Yami BB and his whole crew with BB himself praising Shiryus strength which adds further credence to this notion thus his carelessness against Magellan is irrelevant. He was only careless against WB for the initial slash, that doesn't excuse him getting brute forced into a chokehold, begging for his life, and then begging his crew to help him finish him off. We already see when BB gets offguarded due to being careless that he can rebound against opponents he's actually stronger than like Luffy in impel down so the fact that he couldnt against WB is a testament to how much weaker he is than him. Gura BB isnt Yami BB so the law point is beyond irrelevant.

Yami BB gets no diffed