The starter should spit the "bendix" out and you should be able to hear that.
The "Bendix" on a stomp starter is all mechanical. It should be possible to run the starter without engaging the drive gear by pressing the button without pushing the starter pedal (which apparently hasn't been tried). This should be a 5-minute diagnosis job for someone who can run a voltage drop test. The circuit is as simple as it can possibly be.
Jerry