It is certainly not reasonable in these circumstances to require the full-time equivalent of nine sessions of cover if the sick partner receives only a half-time share of profits.
Indeed, to do so could amount to a fraudulent claim against your insurance company if you continued to claim payments for nine sessions.
Under the policy terms, the company is no doubt only required to fund the number of sessions that you are 'missing' in income terms as a result of your sickness. Otherwise, the effect would be to allow the other partners to receive the 'released' half share of profits, without taking responsibility for the cost of the clinical time associated with that share.
The fact that there is no partnership deed, and your partners are seeking to rely on a verbal discussion you cannot recall taking place, complicates matters.
If you cannot resolve the issue amicably, a partnership split may well occur. If this happens, the PCT could terminate the practice's NHS contract as the partnership would have ceased to exist. All the partners would then lose out.