Regarding the dropping of WP smoke directly on the enemy...
I've not tried that specifically, but when firing my own misdirected (poor los to target) 75mm smoke barrage, the rounds started falling right on my forming up units and then proceeded to walk all over my scattering troops. The casualties were horrendous. One squad reduced to a single man and several others decimated. Approx 30 men lost in all.
Needless to say, I'm now much more carefull about getting a good line of sight to the target area and the company commander who directed this massacre has now been sent to guard my lorry park and think about what he's done.