I've run into the "invisible portal/mob" thing a couple of times (mainly in arena 5) and reported it, but no word from ZOS on what was causing that glitch. The last time it happened, I had to leave and reset the arena completely. Regarding the "invincible" mobs on 7, the reason you're running into that is because you're fighting them in the green pool. Once you pull them out, they will take damage normally.
I have the same issues on random arenas. I have been killed by a dead crematorial guard with solkyn at 1%. 5 sec after he died I got robbed by a melee attack. I have lost interest in doing this. The game has robbed me on the last boss nearly ten runs. I even got to him with 485 lives so I know it's not for lack of learning the dungeon or lack of skill.