This doesn’t affect the argument. Rats could have “consistently” chosen fatty food without choosing it every single time.

This is irrelevant. The author makes no claim about fat inside the rats’ brains—only the fat in their food and the galanin in their brains.

This doesn’t mean the fatty and lean diets contained similar amounts of galanin. This is fully compatible with the reverse causation: the rats who preferred fatty foods simply consumed more galanin in their diets.

This strengthens the argument by casting doubt on an alternative explanation. It makes the reverse causation—that rats had lots of galanin in their brains because of their high fat intake—less likely.

This detail is compatible with the conclusion, but it doesn’t strengthen the argument. It’s just as compatible with the reverse causation: rats who metabolize fat less efficiently crave fattier foods, and that higher fat consumption causes galanin to build up in their brains.