Hi Sir Monster,
If you are getting a modmod ready, please post it here. There's been a lot of discussion about how to improve (if improvement is needed) the tech tree for the first few eras. The Medieval Era in particular needs some work, and we will probably be expanding that at some point.
Traditionally the term "Prehistoric" has meant "before Writing", though I think the word is falling out of use in academic circles. The way we use it, "Prehistoric" means roughly "before Agriculture". It's already a long era, so I would be concerned that your changes would be making it too long unless we split it up.
Gunpowder could arguably be in the Medieval Era. I think Oil Painting, which became common in the 15th century, belongs in the Renaissance; indeed, it is probably the best tech we have that captures Renaissance art. For Divine Right, I think the tech refers specifically to the theory of divine right of kings as presented by Jean Bodin, so that belongs squarely in the Renaissance, but there are other, older concepts such as the Mandate of Heaven to which this could refer. As for the other changes you suggest, I don't have strong views on way or the other.