1. More or less likely than whom? Whether parents who keep children out of school enhance or degrade mental development relative to children who attend school depends on the parents and on the schools that the comparison group attend.
2. That depends on the school.
3. They won't be answered "yes" or "no" because, like the question "does X-radiation enhance longevity" the answer depends on the specific case. Radiation therapy is not indicated for the common cold, for example.
4. The relation between credentials and income indicates the success of the rent-seekers political strategy of lobbying for degree requirements for various employment and against use to pre-employment tests.