Disconnect everything and connect a known keypad on a short wire from the panel. Prove those 2 items work and add things. Keep testing as you add, then you will find the first failing item
wow they are old units.
Its probably your switcher, relay failed.
Still available to swap. test by putting the ibx in manual overide. If no 240v output on the light side then thats ur issue
No you would need a control panel and an rf smoke detector. I don't know of one with just a slave sounder as it would need to poll etc to prove the integrity of the wireless link
The old modules are obsolete and without a picture its difficult to be 100% certain of what you have
But this is what i think you need
https://www.securitywarehouse.co.uk/catalog/videx-8000-series-amplifier-module-1-button-p-3247.html