Ok, I am addressing all issues relating to your earphones having weird problems. If you are not using original Apple EarPods, the issue is your headphones, many pairs of third party earphones i used have this is issue, i can confirm this issue is caused by Apple. Any other earphones that has 3 stripes on the 3.5mm audio jack, (2 lines are for stereo audio, the third line is for mic or remote's buttons). the buttons(volume, play/pause) will not work if they are not Apple Earpods or earphones that are sold to be used with Apple products. (Samsung's earphones and some earphones I used have non-functioning remotes when connect with iOS but works with my Samsung phone)
Second issue is the multiple volume issue, the system will automatically saves different volumes for different audio output category. Device's speaker, headphones, earphones, wireless ones, Airplay and more.
Third issue is starts playing by itself thing, it happens to 2 pairs of my earphones. If you are using a pair of third party headphones with in built remote or mic functions, this might happen. I think it's cause by the dac or whatever driver headphones uses to sort out the remotes signal. The self playing error occurs becasue your headphones cannot communicate the play pause signals with your device properly. (Siri is activated by long pressing the pause button) Again, it's likely to be cause by incompatible earphones. Pressing or bending the cable hardly can also cause abnormal self play pause to occur.
My results comes from using about 10 different headphones and earphones with my iPad and iPhone.