To your first point you are right. Since you know where the tether will be you can consistently pull Exdeath towards the first tether. Now imagine if it could just stay there 100% of the time since the tank would never take the following tethers. This sounds pretty consistent to me good point so double tether is more consistent for that part.

Next point, you have never died to the auto and the gap is "gargantuan". The delay from the 1 HP to the auto is literally 3-4 seconds. But I'm glad you think 3-4 is gargantuan, I'm sure the women in your life agree.

And finally, moving out with TIL is a non issue. By your logic I am assuming you are moving the boss to where the whole party is already going? Good keep doing that and at the same time the DPS who now takes that last tether can do what you do and go with you. If it is simple to know which side the entire party is going to dodge Kefka then they can go that way as well. So double ether by your own admission will not lose anyone uptime