I had the exact same problem. From one day to the other my alert slider would not work. It started with continuous vibrations when my phone was constantly switching profiles. It ended with being stuck in “Do Not Disturb”.
When I opened my phone, the little part of the slider was almost broken off the motherboard. It seemed the soldering joints were completely off and not holding the Alert Slider anymore.
For me soldering worked, a friend of mine tried soldering everything together and after some tries it worked again. At some point only Do Not Disturb-Mode and Vibration-Mode would work, so we had to try again. Be careful as those soldering joints are very close together.
I think there is quite a chance to get your Alert Slider back to work, though I am not sure how long this repair will last.
BTW: After comparing your picture to the pictures of the OnePlus2 Teardown, I don't think that there is something missing on your motherboard. One can spot the little slider on the bottom right.
Also be careful with the position of the slider before removing the motherboard. It can only be removed safely in top position (in Vibration Mode)!!