If those who retire before October 2023 are not given a choice of benefits for period 2015 to 2022 until some later date, what pension payment do they actually receive at the point of leaving the service?

If you retire before October 2023, your benefits will be processed as they are now. For example, if you were all a fully protected member and won’t move to alpha until 1 April 2022, all the benefits you’ve accrued until 31 March 2022 will be in your previous PCSPS scheme and benefits accrued from 1 April 2022 will be calculated in alpha.