M4 MacBook Pro - Keyboard and Trackpad Unresponsive

You could try a DFU Firmware Revive which resets the security enclave chip & system firmware to see if that makes any difference. Unfortunately this requires access to another Mac running macOS 15.x Sequoia (at least at this time).

When you restored the OS, did you just reinstall over top of itself, or did you perform a clean install by first erasing the disk (aka deleting the Volume Group)? Reinstalling macOS over top of itself doesn't help things as much as it used to do since the OS is now on a signed & sealed APFS volume. At best it may help with a boot issue. The best way to confirm a hardware issue is with a clean install of macOS & testing thoroughly before installing any third party apps and before restoring from a backup, otherwise you may be bringing the problem back. A DFU Firmware Restore is a quick way to do a clean install and it will also reset the security enclave chip, system firmware, and internal SSD at the same time.

I don't know how the keyboard works on the M-series Macs, but with recent Intel Macs since 2015 the Keyboards are accessed using USB when booting the firmware before macOS and using SPI once logged into macOS. So if it works the same on the M-series Mac, then it can indeed be a hardware issue where the one access method has a problem. I've seen bad Trackpads, Trackpad cables, and bad Logic Boards cause this problem on the 2015+ Inte4l Macs.