let me suggest something fair...
to safe guard against all parties, contract is necessary.
For Organiser & Photographers, Payment from Photographers = acceptance of the contract
If Photographer don't show up, its their own problem.
If Organiser don't show up, then Photographers can request for reshoot or appropriate resolution.
If there is any changes from Organiser side, minimum of 24hours before shoot in notification.
For Models & Organisers = Payment to Models confirms the shoot.
If Model don't show up, they breached the contract, hence liable to pay for the cost of organiser/photographers/etc.
If Organiser change details in last minute, also breaches the contract. Hence a proper listing of duty to be provided to the models. Both should agree on the things to be done, level of difficulty of task to be performed by models to photographers. Also, this list should be briefed to the photographers as well so that they know not to overstep their authority.
If models decide to pull off from the shoot, a minimum of 48hours notification prior to shoot to allow time for organiser to find replacement and negotiate with photographers. Photographers can then decide if they want to shoot the alternative models or a refund.
Anyway above is just a suggestion. Organiser job is not easy, i understand, but overall, everyone is here to have fun.