Sure Jesus is called Mighty God in Isaiah. If you read context of the verse, this was titles given to him due to his role as King of New Jerusalem.
You can't be given titles such as Mighty God if you're God.
Now, what do you think of this verse?
Now this is eternal life: that they may know you, the only true God, and Jesus Christ, whom you have sent.
Why does Jesus call his Father the only true God? Can it be more clean than that?
Both scriptures state that the Father is the only true God. Based on the context of 1 John 5:20, Him who is true is the Father NOT Jesus.
No, it doesn't revert back to Jehovah - The statement "Him" is God, and when it states "His", it means its referring to God's son.
So again, Is Jesus correct by calling his father the ONLY (only meaning no more than one) true God?
1 John 5:20
We know also that the Son of God has come and has given us understanding, so that we may know him (THE FATHER) who is true. And we are in him (THE FATHER) who is true—even in (en 1722 Greek) his Son Jesus Christ. He (THE FATHER) is the true God and eternal life.
Greek - 1722 - en
a primary preposition denoting (fixed) position (in place, time or state), and (by implication) instrumentality (medially or constructively), i.e. a relation of rest (intermediate between eiV - eis 1519 and 1537); "in," at, (up-)on, by, etc.:--about, after, against, + almost, X altogether, among, X as, at, before, between, (here-)by (+ all means), for (... sake of), + give self wholly to, (here-)in(-to, -wardly), X mightily, (because) of, (up-)on, (open-)ly, X outwardly, one, X quickly, X shortly, (speedi-)ly, X that, X there(-in, -on), through(-out), (un-)to(-ward), under, when, where(-with), while, with(-in). Often used in compounds, with substantially the same import; rarely with verbs of motion, and then not to indicate direction, except (elliptically) by a separate (and different) preposition.
“we are (2070) (5748) in (1722) him that is true (228), even in (1722) his (846) Son (5207) Jesus (2424) Christ (5547).”
You can try all you want, but its not going to change the fact that this is referring to the father as him being the true God, not Jesus. The point is not weather kai is being used.