Ace,

Crying babies are part of life. The fact that the mother looked like she wasn't concerned is probably true. Maybe there was no reason to be concerned. If she knows her baby, maybe nothing was wrong except teething, or maybe an ear infection that prevented the baby from equalizing properly (imagine that and a tooth problem while doing a deep dive). They say that teething is one of the most painful human experiences part of regularly scheduled life next to giving birth. Adults would not be able to handle it without serious drugs.

Look, it is annoying, I wouldn't like it at all, but don't you think that the mother would like some quiet as well as not being looked at with scorn by all on the plane?

What could the mother have done? I'm sure if the mother tried gagging or placing in the cargo hold like you suggested, you'd be on telling us about the abuse.