Up to a certain age. By 3 or 4, adults should be speaking like adults so the child will learn that. I think continuing past that point could even harm the child psychologically.

To a degree. O did a bit 30 some years ago. Nothing extreme.

If they are very young. Motherese helps language, doesn't hinder.

Of course. They don't actually understand the words but they love the tone. They will pick up the language as they grow olderf.

Baby talk as in baby tones and sing song voices is fine. Just use the correct words.

Sometimes it helps to get to the child's level

This all depends on the age of the child.

Depends on the age but overall why not?

Yes it's perfectly normal, they are children, so let them be children, don't expect too much out of children, it takes away from their childhood

Only when they are infants.