1) yes and this happens even if you run away further then 0.5 cast. When we were progressing this boss i was getting really frustrated with this mechanic that seem to be broken. Look at the short video below and you will notice our sentinel getting pushed back into stun even though he was past sage healer that didn't get stuck:
2) Don't stand on the corner. Move inside one of the wall so you are already safe of have to move to other side. As soon as you see IMMUNE message to your damaging skills (if you do DPS) then look at mini map. RED DOT indicates where he will land few seconds before. Look at our kill video from around 5:15 and you will see I got all people to stand next to me (i marked myself) then i soon as i saw red dot in the same line as we were I told people to move to other side.