Dr. Craig is a liar.

[Mister Agenda]

Question: How long was this period prior to the creation of time in which God existed in a timeless, changeless state?

I think the answer has to be that the proposed prior existence would be instantaneous. Without time, you could say 'everything happens at once'. There would be zero time between the 'timeless state' and time beginning. As you said, that makes God's beginning the same instant as the universe beginning; and God is scientifically superfluous if a quantum vacuum fluctuation can do the same job.

Which brings me to the tangential thought that the God of the KCA is a job description more than a being.