I am not an expert on what is in the public domain, nor an expert in the license, but what you say Urizen is only partially true.
Hell and some of the named archfiends are derived from Dante's work and other sources that Wotc does not own. However, there are some tricky things. For example the 8th layer of hell in the DnD universe is called Cania. While in Dante's Inferno there was a region named Caina. Why Wotc made this change, I have no idea but could be the same for the heucuva/huecuva (however it is spelled) example.
Other example; the Hag Countess is off limits, as she's an invention of Wotc. However, Mammon is fine to use, that name existed in early texts as a force of evil. The same goes for other names and places.
Olympus, most commonly associated with Greek mythology is fine to use, because Wotc didn't invent it.
So, anything you can find from mythology and public domain can be used in your new setting.