Being clever after the event here, but.......
A red knob for mixture ?? All our lives we have seen red on water taps and car heater controls; meaning heat or hot.
So a student, new to this type, with a hundred other things on their mind is reminded to apply heat, while also flying and controlling the aircraft in the descent.
Their conscious processing capacity is completely used up, with no spare capacity, so they use their deep memory and reach for the red control which in their deep memory means heat. Even though they were briefed about the mixture control.
And Robinson have apparently known about this problem for years. So why therefore haven't they - and other piston engined aircraft manufacturers - changed the colour of the mixture knob ? Black and white stripes, for example.
Having a normal flying control coloured red is a bit odd anyway - red controls are generally for emergency situations, such as engine fire.
Luckily both occupants were not injured, owing to an excellent recovery by the instructor
Regarding the A109 rotor brake; surely that is fixable by following a pre-start paper checklist ?.