This is irrelevant. The argument has already established that plants absorb heavy nitrogen, so the exact source of the heavy nitrogen doesn’t matter.

This is irrelevant, since the argument doesn’t make claims based on the rate of accumulation of heavy nitrogen in tissue, only the concentration of heavy nitrogen.

This is irrelevant, because the exact number of samples doesn’t really make a difference. Either there were enough samples to be representative or there weren’t—either way, it would be equally possible to have the same number of samples.

This strengthens by providing a closer comparison between cave bears and modern bears. If modern bears’ heavy nitrogen levels are identical between blood and bone, it’s more reasonable to draw conclusions by comparing cave bears’ bones and modern bears’ blood.

This is irrelevant, since we can already be confident that the heavy nitrogen level in the modern bear samples is representative of a diet that includes meat.