This does not weaken the argument. Whether or not there’s a genetic factor to colic is irrelevant to whether the antibodies in cow’s milk can cause colic. Plus, this doesn’t even establish a genetic factor without more context about colic rates outside of twins.

This does not weaken the argument. Whether or not colic goes away as children grow up has no bearing on what the initial cause of colic could be. This has nothing to do with any possible link between colic and cow’s milk antibodies.

This does not weaken the argument. Undermining the link between cow’s milk and colic would strengthen, not weaken. We also don’t know the overall likelihood of an infant getting colic, and without context these numbers don’t actually mean much either way.

This weakens the argument. Now we have a link between cow’s milk and colic, even in infants who only drink breast milk. This undermines the support provided to the author’s conclusion that cow’s milk antibodies probably do not cause colic.

This does not weaken the argument. It doesn’t matter if the infants who are only fed breast milk can tolerate cow’s milk or not: they’re not drinking cow’s milk, but they’re still getting colic. This doesn’t interfere with the link between the author’s evidence and hypothesis.