Unfortunately it isn't possible to make someone else behave reasonably, all you can do is adopt strategies to cope and minimise any impact on the children. To do that you need to focus on your behaviour eg establishing physical and emotional boundaries, not reacting to negative things your ex says or the children report he has said, generally teaching the children to see things for themselves rather than believe everything they are told.
The first couple of years after separation and before arrangements for finances and children are properly sorted can be chaotic, but things do tend to get better in time. Its a pain, but if you have agreed arrangements for him to have the children Wednesday and Friday I would just accept he is going to be unreliable and make contingency plans so the impact on the children is minimised.
As far as child maintenance is concerned it is paid to the parent with the majority of care for the benefit of children as a contribution toward the costs of raising children. You can agree a family based arrangement between yourselves. The CM Options website has a lot of information and tools to help parents make their own arrangements.
Experience shows parents are more satisfied with arrangements and more likely to adhere to them if they are agreed. It may help to seek agreement for a written parenting plan and family based agreement so you both know where you stand. Conditions about a reasonable period of notice to change contact can be included.
Alternatively you could apply to the CMS for a calculation and then you can claim any arrears, either parent can apply to court for an order regulating living and contact arrangements and applying to court for a financial remedy could help focus minds. However good contact for children relies on separated parents learning to work together, or at least not against each other. Going to court tends to leave people feeling resentful and resistant making working together in the future difficult if not impossible. Therefore court really should be the last option.