"Good point! An actual "witness" has to have seen something that they can testify to."
Yes, and that is actually the point being made at Isaiah 43 where Yahweh calls the Israelites his witnesses. He was telling them that they are witnesses to the fact that he is the only real god because they experienced his miraculous saving acts in their behalf. A contrast was being made between Yahweh, the only true God who acts (which the Israelites could testify to), and the false gods of the nations that do nothing.
So what Isaiah 43:10 says only applies to those who witnessed the reality of Yahweh's godship by experiencing his miraculous saving acts. By coopting Isaiah 43:10 out of this context and applying it to themselves, the JWs are twisting the scripture and misapplying it. They are clearly not Yahweh's Witnesses since they, as a group, have never experienced any miraculous saving acts by Jehovah.
Great points made in the OP too. But JWs are not open to reason. I think they try to get around the points by saying that when the bible talks about an evangelizer it is not referring merely to someone who preaches the good news (even though that is literally what the word means. lol) but that it refers only to special, full-time preachers.
Dishonest Watchtower. If they're not twisting the meaning of scriptures then they're twisting the meaning of words.