I don't think that a contract of retainership should have a clause stating that leaves, insurance, etc., should not be included. Of course, there can be reimbursement for certain expenses, such as travel related to the office, fees paid to departments on behalf of the establishment, etc.
Eligibility for Retainership
In my opinion, not all employees can be retained as a retainer. Only those who can provide advice on technical, managerial, or other relevant matters can be designated as retainers. There is no point in engaging a worker or a junior supervisor by simply redesignating them as a retainer. A retainer should not be bound by your office timing, leave rules, dress code, etc.
Duration of Retainership
I believe a retainership can extend beyond 2 years. What truly matters is the role.