Very loose. The finnish equivalent for those words would be “karhumainen”. I’m not sure there is a single-word translation for the “-kainen” suffix.
It doesn’t really work the same way “-mainen” does, it’s similar, but results in a word that has a more definite meaning.
“Lapsimainen” would mean childlike or childish, while “lapsukainen” only ever refers to an actual child in an endeared way (or if used to describe an adult, would be like saying “my sweet summer child” in english).
Close enough, but not closest. I would say “Bearie” is closer. You wouldn’t call an actual bear “bearlike” but you might name it “Bearie”.
I didn’t say “mainen” and “kainen” are dissimilar, but they ARE different. By about the same amount as the difference between doglike/doggy and doggie.