It's probably best to start fresh here-
Post the model # etc. of your tractor. Possibly there is an electrical schematic available.
Describe what happens and what DOESN'T happen.
Does the carburetor fuel solenoid CLICK when you turn the key ON?

There can be multiple "interlock" switches on a tractor.
Seat switch
Clutch/brake switch.
PTO/attachment switch
Seat switch.

The starter solenoid is a "relay".
12V is delivered from the START terminal of the key switch.
Typically, the current will have to pass through the clutch/brake switch and then the PTO/attachment switch before it reaches the starter solenoid.

Your starter solenoid may have 1 or 2 small terminals on it. IF 2, one will be to ground. Possibly through another switch.
The OTHER one should activate the solenoid and crank the engine when 12V is applied to it.
Apply 12V to it and see if the engine cranks.
IF it does, we know you have a good battery and CRANK circuit.
The rest will then be determining which switch is bad or misadjusted between the starter switch and the 2 safety switches.