The very fact that you have asked this thoughtful question suggests you will know when you are in danger of crossing this line. I don't believe that truly manipulative people start from where you are, since your programme involves developing individuals and encouraging group identity, dangerous activities for anyone bent on master-minding outcomes.
A born manipulator would be more concerned to go in for selective provision and suppression of information, promoting confusion of aims and uncertainty about rules and, above all, fostering or even inflaming conflicts and inequalities among the governors.
If you feel threatened rather than pleased when your governing body has become confident to the point of asking difficult questions, challenging deeply entrenched practices and initiating debate, you need to be concerned. But I don't think you will react in this way at all. You will be proud of your success.
Should we always accept a request to go part-time or job share? It seems that it suits a teacher for family, health or study reasons (or just because they have a well-paid partner!) but it is not always in the children's interests.
Your last phrase gives you the answer. The needs of the curriculum and the smooth running of the school - in other words, the pupils' interests - should always guide your decision. But these considerations are rarely so straightforward.
Compassion in the case of health, bereavement, the needs of a new mother, a desire to improve qualifications, should weigh significantly with a good employer and therefore with governors. A good teacher may give better service if not under strain, and often part-time and job-sharing staff represent excellent value for money.
Above all, there is also an equal opportunities aspect, since many staff who want part-time work will be women: their needs should be considered by an equal opportunities employer, if they can be reconciled with pupils' well-being. Even your heart-felt remark about well-paid partners has another side: in high-cost housing areas schools have reason to be grateful to well-paid partners for enabling them to get enough teachers at all!
Much will depend on the job needs in the particular case. In a small primary, a part-time teacher responsible for technology or music throughout the school might be ideal, but perhaps a job share of class-teacher might involve more pros and cons.
In a secondary school, timetabling problems and the strengths and work patterns of the other staff in the subject would play a part.