Interlock switch ?
you have a seat switch
You have a brake switch
You have a PTO switch
The brake switch is working because you can start the mower with the brake on.
So I would be looking at the PTO switch.
Can you engage the blades with the engine running if you keep your foot on the brake ?
If the engine stops or the PTO does not engage ( Electric PTO ) then the seat switch , or the wires to or from it are bad.