Regarding Isaiah 43:10 I believe that context is important to show why God said this part of the verse, "Before Me there was not God formed, And there will be none after Me."
The following is a summary of what Albert Barns says on the subject.
I. In the previous chapter he had severely rebuked the Jews, as being deaf, and blind, and had showed them that it was on account of their sins that these calamities had come upon them. Yet he now turns and says, that they are the people whom he had redeemed, and whom it was his purpose to deliver, and repeats the solemn assurance that they would be rescued Isaiah 43:1-7. This assurance consists of many items, or considerations, showing that they would be recovered, however far they were driven from their own land.
God had formed and redeemed them Isaiah 43:1. It followed from this that a God of covenant faithfulness would be with them in their trials Isaiah 43:2.
They had been so precious to him and valuable, that he had given entire nations for their ransom Isaiah 43:3. It followed from this, that he would continue to give more, if necessary, for their ransom Isaiah 43:4.
It was rite fixed purpose of God to gather them again, wherever they might be scattered, and they had, therefore, nothing to fear Isaiah 43:5-7.
II. God asserts his superiority to all idol-gods. He makes a solemn appeal, as he had done in Isaiah 41, to show that the idols had no power; and refers to all that he had predicted and to its fulfillment in proof that he was the only true God, and had been faithful to his people Isaiah 43:8-13. In doing this, he says:
That none of the idols had been able to predict future events Isaiah 43:8-9.
That the Jewish people were his witnesses that he was the true God, and the only Saviour Isaiah 43:10-12.
That he had existed forever, and that none could thwart his designs Isaiah 43:13.
Isaiah 43:10, the first part is explained. Ye are my witnesses - They were his witnesses, because, first, he had given in them predictions of future events which had been literally fulfilled: secondly, by his power of delivering them so often manifested, he had shown that he was a God able to save. Neither of these had been done by the idol-gods (compare Isaiah 44:8).
Isaiah 44:8, "Do not tremble and do not be afraid; Have I not long since announced it to you and declared it? And you are My Witnesses, Is there any God besides Me, Or is there any other Rock? I know of none."
Now, it is a fact that has been established there is only one God and there are no gods formed before Jehovah and none are formed after Him. Yet the JW's insist that Jesus Christ is "a god" according to the NWT's rendering of John 1:1.
Moreover, at Philippians 2:6-7 the Apostle Paul says the following: "who (referring to Jesus Christ), although he was existing in God's form, did not regard equality with God a thing to be grasped."
That word "although" means, "in spite of the fact or granting that." This verse makes it clear that even though Jesus Christ possessed equality with the Father, Jesus did not cling to it. Cling to what exactly? He did not cling to His prerogatives as God's equal.
Verse 7, "but emptied Himself (of His divine prerogatives, not His attributes) taking the form of a bond-servant (by consenting to be a slave) and being made in the likeness of men" John 1:14, "And the Logos/Word became flesh, and dwelt among us and we beheld His glory, glory as of the only begotten from the Father, full of grace and truth."
The Apostle Paul is "NOT" advocating that God is designating Jesus as His representative or spokesman in these verses. This is the position the JW's are taking.
So in conclusion, we know from Isaiah 43:10 that God said, "Before Me there was no God formed, And there will be none after Me." Yet you have the Apostle Paul saying Jesus Christ always existed in the form of God, (not "a god") and Jesus took on another form, that of a human being. For the JW's, if according to you Jesus is "a god," is He a true God or false God?