Can the resistance of the dynamic head support hardware be adjusted?

The elastomers can be switched to change the level of resistance. The hardware comes standard with Blue (medium) elastomers to provide movement and to return the client to a starting position. If more resistance is needed, Green (firm) elastomers are provided with the unit.

https://www.seatingdynamics.com/2018/03/06/determining-resistance-dynamic-seating-head/
https://www.seatingdynamics.com/2018/09/04/dynamic-head-support-tilt-and-recline-resistance/

Which dynamic head support hardware should I recommend?

The Single Axis dynamic head support hardware provides approximately 8 degrees of movement posteriorly. The resistance can be changed using different elastomers.

The Multi Axis dynamic headrest provides movement posteriorly and laterally, allowing this to respond to rotational movement. If the client does not always extend directly in midline, but has some rotation to their movement, the multi axis dynamic headrest is indicated.

https://www.seatingdynamics.com/2017/03/06/dynamic-head-supports-importance-design-meeting-client-goals/