Myth of Jesus, page-655

Further back in John, Jesus gives quite a message and more than once that the Father dwelt in him, and it is He that does the works.
Thomas is part of the clan that received this message, so as you say, the Key, Jesus is alive, Thomas's doubts gone and obviously would remember, God is within Jesus, not to mention Jesus making the repeated statement I can do nothing of myself, it is the Father within me.
Hence Thomas thinking God with in him and He being the one that made Jesus alive and says "AND" My God.

I like and envy your eye for detail.