As for Urouge and Kidd, yeah I wonder why their relatives aren't being assassinated too… maybe we will have an explanation. But at least, an entire arc wasn't triggered on that said threat...
Anyway, something like "Urouge's relatives couldn't be found because of sky islands "issues" " and "Kidd is a monster who doesn't have any relatives but his crew" would do it for me.
I mean if she went out and beat up Kaido or Shanks grandparents would that make her more feared?
I think the yonkous can protect their relatives
Daz is completely right on this issue. It wouldn't even be that hard to write a better solution to this dilemma. The Strawhats could have tamed BM by baking the best cake that BM even had eaten in order to at least negotiate some kind of partial truce. That'd also fit thematically perfectly as it would prove the point to the Vinsmokes that cooking not only is not useless, but also is the only saving grace for them whereas all of their solutions blatantly fail.
Alternatively the Strawhats might have tried the figure out the source of the network within the boundaries of the OP world. So the BM pirates can easily travel to whatever place in the world. How are they doing this? Huh, they have access to some kind of magical dimension which connects every mirror in the whole world. Great, we found out, so let's see how we can disable this…
Honestly, during the entire arc I was expecting Luffy to call his fleet and set a strategy. Maybe not Luffy himself, but one of his crewmates