A safe harbor match can not require hours in order to receive it on a year-by-year basis, but it can have a service requirement for initial eligibility.
No, you can have different eligibility for deferrals and safe harbor match. The major consequence of this design is the loss of the top heavy exemption, as Bri noted earlier. Under this design you are technically doing an ADP test for the disaggregated portion of the plan covering otherwise excludable employees, since that group is not covered by the safe harbor match. It is unlikely that there would be any otherwise excludable HCEs, so that group should always pass the test automatically. But it's something to be aware of.
No, you can have a service (hours or elapsed time) for initial eligibility for matching contributions, including safe harbor matching contributions. If your document uses a checkbox-style adoption agreement, there are probably options for this.
A plan that consists solely of deferrals and matching contributions which satisfy the ADP and ACP safe harbors is exempt from top heavy. This is determined based on the contributions that are actually made to the plan on a year-by-year basis. A plan can permit non-elective contributions but will not lose its top heavy exemption unless non-elective contributions are actually made (or forfeitures allocated) in a given year. Likewise, making non-safe harbor matching contributions will also cause the plan to lose its top heavy exemption.