Why do Jehovah's Witnesses not believe in a literal, visible parousia of Christ, since..?

You're making some strange inferences from the phrase "whom heaven, indeed, must hold within itself." I'm not seeing how it necessarily implies invisibility. That would be only one possible interpretation. The obvious interpretation would be that Jesus will stay in heaven until he ready to return. His appearance is not mentioned in the first citation. The second citation simply implies that he will be visible when he returns. In the big picture, I'm not sure why it matters how Jesus looks in heaven. In my opinion, arguing with JWs is always a waste of time. They are trained to wear you down and irritate you. Taking a more virtuous approach is within your grasp.