Easy for an adult, not so easy for a baby. Bilabial and coronal sounds don't take much tongue dexterity: you just need to be able to open/close your lips and leave your tongue in its natural resting position respectively to produce them. Velar sounds, on the other hand, require you to flex the middle of your tongue to touch your palate, which is as significantly more difficult sound to produce.

It's no coincidence that sounds like 'gaga' and 'kaka' tend to be associated with either more distant relatives or with defecation: the kid's going to be older when those things get introduced.

Why would it be? What's your reasoning for expecting it to be? Did you read the paper linked to?

And correct, as it happens. Though long-running cultural associations with certain consonants (such as 'm', 'n' and 'b' and mother or grandmother, 'p' and 'd' with father or grandfather, &c.) to play a huge role in it too: the parents need to be culturally primed to mistake the babbling for actual half-formed words, and subsequently build that association in the child's mind in return.

I'm not sure why I was downvoted - Larry Trask's article has the exact same explanation i gave above - mainly that the 'm' and 'p' sounds are the easiest consonant vocalizations that babies can do.