On the sick pay issue, the model contract of employment offers an escalating entitlement, based upon length of service, with a ceiling after 5 years' service of 6 months at full pay followed by six months at half pay.
On the wider issues, your council appears to be operating entirely outside the legal framework. I suggest that you discuss this with the Monitoring Officer of your district or borough council and with your local Association of Local Councils. If you did not receive an adequate response to your FOI requests, you should report this to the Information Commissioner.
There will be new elections for your parish council soon, probably in May 2023, so perhaps you should think about encouraging your neighbours to stand for election to replace those currently in office.