May an employer pay 100% of the group health premium for full-time employees, 0% for their spouses and children, and 50% for part-time employees? Also, may an employer vary the contribution percentage for full-time employees based on years of service?

Yes.

[That's the short answer. A short expansion follows:]

An employer can pay all or part of the premium for any group of employees and dependents, except in Hawaii where the law requires coverage and dictates the minimum employer contribution.