The argument doesn't rely on a claim that a porous outer bone shell is necessary for deep dives, so refuting that doesn't weaken. If this were reversed to say that some animals with a porous outer bone shell were not deep divers, that could weaken, but as is it doesn't harm the argument.

We have no idea if those reptiles are deep-divers. Even if we knew that they were, this wouldn't weaken for the same reason that (A) doesn't: the author doesn't assume that porous outer bone shells are necessary for deep-diving, so undermining that idea doesn't make a difference.

This provides contrary evidence which indicates an alternative explanation for the ichthyosaurs' bone structure: it could be for the same reason that most non-deep-diving marine reptiles have porous outer bone shells. This effectively undermines the argument.

All this says is that we don't have additional information; it doesn't say that ichthyosaurs didn't share these characteristics, just that we don't know. Sure, this doesn't strengthen the analogy, but it doesn't weaken either. It does nothing at all.

This tells us that ichthyosaurs could’ve been deep divers even without porous outer bone shells. But we're trying to weaken the claim that ichthyosaurs were deep divers at all, so finding more ways for ichthyosaurs to be deep divers doesn't help us.