I think it should work as follows:
1. If you start the machine it starts in neutral. Brakes must be applied to start and vehicle must be stationary.
2. If you select up shift from neutral it goes into 1st gear. Selecting down shift from neutral would remain in neutral. Brakes must be applied and the vehicle must be stationary.
3. To go into reverse, from neutral, press the reverse button. Brakes must be applied to do this and the vehicle must be stationary. Reverse is then selected.
4. To go into reverse, whilst in gear, press the reverse button. Brakes must be applied to do this and the vehicle must be stationary and in first gear only. Reverse is then selected.
5. To go from reverse to a forward gear, up shift will go into first gear, down shift will go into neutral. Brakes must be applied to do this and the vehicle stationary.
This would all be no logical difference from a car and all quite safe. You would not be able select reverse unless the vehicle was stationary with the brakes applied. No need for two awkward actions to be performed simultaneously, all rather more logical I think.