1. Only "him" 3 (Actually #2). "crack and crevice" is a stable expression, I don't think the words differ much. Maybe a crevice is a narrow crack... 4. Sometimes yes, sometimes no.
«Why can't I use "he" in the second example, but "him" only.»
Because in structures such as "I like him", "him" is not in the nominative case. When it's the target of some verb (like, want...) it should be "him" and "her":