I like wall facing squats to help improve the squat skill/keep the core stiff. It's self correcting as you won't flop too far because there's a wall there.
Weight training with bad form is to be avoided. So I agree with you. but C's skill could be improved very quickly (I think). There's a lot going on with poor form. Some is mobility but how can you tell until he learns how to do the exercise. A simple improvement might give him a better score, but would the mobility change much? Bad foot work, the heal lift, bad shoes, the lack of lumbar curve, the lack of body firmness, breathing, head position, inactive shoulders - among other form issues... he is better than the scale might indicate. Coaching could/might quickly fix all of that and then he can add a little weight.
Wall facing squats are great, they reveal problems with form and mobility.
Just my thoughts... not a professional. We work with this exercise a lot so I see guys like C improve quickly! But does this equal mobility?