Then you have people who briefly slow down for the camera, then speed back up.

Do you want a panopticon that fascists could use to surveil and persecute their political enemies? 'Cause that's how you get one.

If the road is narrow and visibly winding with lots of trees close by, you bet those cars would slow down just to not wreck themselves. All while everyone else can use the road as normal.

I don't understand what point you're trying to make. If they take a wide, straight road that people are comfortably doing 50 mph on and then lower the speed limit to 25 mph to make it "safe" for pedestrians, horse riders, cyclists, etc., people are just going to keep doing 50 mph anyway and mow down those vulnerable road users.

Changing the speed limit, by itself, does fuck-all to make the road more accommodating for non-car users. The road has to actually be made scarier or less comfortable for drivers via things like narrowed lanes, chicanes, speed tables, etc. in order to actually get drivers to slow down to the point where other road users are actually safe.