I think your argument assumes how God would address himself as a 3 Person yet 1 ontological Being... why not "that they may know US, the only true God?" If Jesus at that time simply wanted to leave out the emphasis that he was God he is not obligated to say it; nor are the Persons of the Trinity obligated in one way or another to always use certain pronouns.And this is eternal life, that they know you the only true God, even Jesus Christ whom you have sent.
I think this is a grave mistake. Jesus was recognizing his Father as the only true God, not declaring Himself to be the only true God. If He had meant the latter, He wouldn't have addressed "the only true God" as "you." Rather He would have said: "... that they may know ME, the only true God."
If we take these premises:
1. Every one of the Trinity is fully the only true God.
2. The Trinity together is also fully the only true God.
Then we can make the following true statements:
The only true God has sent the Holy Spirit.
The only true God died on the cross in the purpose of the Father.
The only true God sent Jesus Christ into the world.
The only true God came into the world in the Person of Jesus Christ.
Assuming the above 2 premises it doesn't logically follow that any of the following disprove them.
I do agree that "kai" as meaning "even" is a stretch there (however there really does seem to be places it's used, such as "all sins will be forgiven men KAI whatever blasphemies they may utter" where it seems this narrows down the intent of the all sins in light of the topic of verbal blasphemy and not just all sins of all time).