No, that's a separate issue. You can upgrade a M4 or earlier machine from 15.6 to 15.7 even today, despite 26.0 being out for a while, so Apple's still signing a 15.x release at the same time as they're offering 26.x releases. (You likely won't be able to downgrade from 15.7 back to 15.6.)
Downgrading a M5 machine to 15.x would be impossible not because of a signing issue but because Apple never released a 15.x build that supported M5 hardware.
No, that's a separate issue. You can upgrade a M4 or earlier machine from 15.6 to 15.7 even today, despite 26.0 being out for a while, so Apple's still signing a 15.x release at the same time as they're offering 26.x releases. (You likely won't be able to downgrade from 15.7 back to 15.6.)
Downgrading a M5 machine to 15.x would be impossible not because of a signing issue but because Apple never released a 15.x build that supported M5 hardware.