Force damage extends from the prime to the etheral, always did, always will. If you go to the border etheral, you can only cast force damage into the prime. You lose your connections to anything that is left on the prime. So your insect plague would not move. I would even rule that the insects just leave the area as you have no way to maintain the connection to control their cohesion.
You would also have disadvantage on your perception to find the wizard as what you see on the prime is hazy and blurred. If you say you go to deep etheral to avoid force damage retaliation you lose the duel. You left the arena.
Claim it with biased and false truths as much as you want, you will still lose to the diviner. You would also stand a losing match against the champion above and even a monk with way of the open hand, mobility, sentinel and mage slayer stands a good chance to blow you to smithereens where the diviner will only yawn as he saw the future. Portent for the win.
What you propose is not bad. But it is a white room. We did it. I told you. The diviner won every single match (to my utmost and absolute dismay). Experience is way better than white room and conjectures. Experience proves you wrong.