I assume it's the same for the mixture friction wheel as well.
Yes Mate, precisely. I am not sure on the original whether the lever for the throttle (which has a "wheel" at its base) and the other friction wheel are connected or separate, the drawings are ambiguous. Given the "two position" nature of the mixture, locking THAT would not be such a bad idea but I doubt it was like that simply because the mixture is automatically pulled back to full rich when the throttle is retartded. If the mixture were locked and you needed to chop the throttle quickly (emergency) the mixture lever would likely jam teh throttle at half open..not good.
throttle changes should never be made on the Merlin except
for in an emergency...usually it is best to keep a nice even movement in both directions. Throttle Friction aids in this by finding the right balance between "firmness' of response and ease of movement.