@dnedeltchev, first of all regarding the RGB bulb recognition, you want to check this topic.
Then you want to change the DH for the bulb to Zigbee Dimmer, the you will not have the color control showed anymore.
Can you please provide some details that which model is it actually? A link or a correct model number would be good. But is it dimmable only or can you change white color temperature too?
What you are explaining in the first part, it is probably due to the motion sensor. These motion sensors are PIR sensors, they detect heat radiation (Passive InfraRed), actually the change in heat signatures. All these devices just to reduce battery consumption they tend to have blind periods after detecting motion. So you might see the effect of PIR, as the heat signature hasn’t change too much and the blind period too. You can check in the app what and when the sensor detects as motion, in the device page.
The second half of your issue is a good question, but I have a guess, as I have seen some similar complaints.
So when you defined the rules for the Hive Spotlight, you defined that you want to set brightness and turn on in the Smart Lighting SmartApp. Try to define a scene first with those settings and add the Scene only to the Smart Lighting rule, does it work that way? If yes, then there might be a bug in Smart Lighting.
(Are you using the new or the Classic app? I guess the new as a new v3 owner.)