Yeah, it is difficult for me to explain so I'll try again, queeq if you read this please translate also
If you look at him when he is on the pile, you notice that the mask is almost fallen in the mask, it doesn't lay on the mask.
Wat ik bedoel is dat het masker ingevallen is, het ligt niet OP het masker maar IN het masker
I believe that when they die then disappear is because they were one with the force. If they die like Qui-Gon did he was surprised by his impending death. So he couldn't get into synch with the force. Evidence Obi-Wan knew let himself be struck down he knew he was going to lose anyways. Yoda died slowly, he knew he was going to die. Vader knew he was going to die.