And then again, would it be wrong to go with "Factually, they don't know where their 'miracles' come from"?
From what I get is that you wish to use all D&D classes and put them into a scenario of Medieval Europe at 1100 AD which is about to be visited by Renaissance-Era elves from the Americas.
So you don't really need a cosmological explanation for the source of divine spells. They just believe that God (or gods or forces of nature) grant them. Some of them think that a form of primeval evil exists and call that "The Devil" or "Satan" and get powers from that source. However, no one knows for sure.
Angels and Demons can exist in your scenario, depending on whether or not you use a planar system. They are forces of good and evil which intervene when they see it fitting.
(and as a sidenote: Be careful with that whole Venus & Mars stuff, interesting as this may be. You're just opening another big can of worms by making our sister planets not only habitable, but inhabited. This, and the existence of actual, powerful romano-greek gods (or... aliens?), changes a LOT from the 1100 AD you'd like to portray.)
I think the gods Venus and Mars are consumed by maintaining their own planets, they have little time an energy left over for dealing with Earthlings once they stopped believing and worshipping them. During their heyday the Roman gods got associated with the planets Mercury, Venus, Mars, Jupiter, and Saturn, but only Venus and Mars are capable of being habitable. Venus is a very old goddess, there are versions of her that existed prior to Roman times, under different names such as Aphrodite and Isthar, and even as far back as the stone age, Mars is a someone younger god but still very old, he goes back to before the Romans as well. An aspect of Venus still exists under the name "Freya" in the Norse pantheon, so she still has a little bit of a following on Earth, Mars goes under the alias of "Thor" as that is effectively the Viking God of War.
Because Venus and Mars have large populations of humans that worship these two deities, they are still active. People on Earth who still worship Venus and Mars, usually keep that fact a secret, lest they be burned as witches, this is true in central and southern Europe and in the Middle East, in Scandinavia, this is tolerated a bit more as that region is still in transition from paganism to Christianity. Lots of bloodlust and warfare still keep the god of war going, even if they are fighting over Christian causes. In some respects, one can say, they God of War managed to subvert the Christian Crusade against Islam, by getting the Crusaders more interested in looting and pillaging than in conversions! Mars loves battle for its own sake, he doesn't care about the underlying reason for the conflict! Venus is much the same but with love, she finds Priest's Monk's vows and chastity a bit of a challenge for her, and she loves to test their faith! Sometimes clerics of Venus and Mars will masquerade as Catholic Priests to advance their true devotions, these pagan deities will grant their spells, thus allowing them to blend in with their true believers.
Usually its bloodlust which allows Mars to corrupt militant Catholic Priests, Mars will often answer their prayers instead of God, if they are really determined to carry out their battles against the Muslim Infidels, and you know Venus can cause a priest to meet a beautiful woman, and maybe she can get something started between them in secret. Priests that give into their temptations with a woman, may find their prayers sometimes being answered by Venus and they might fall into her orbit, while at the same time maintaining their positions within the Catholic Church for as long as they can get away with it. Venus often helps them to maintain their secret affairs for as long as possible, she loves illicit affairs, having had a few of those herself.
As for the other two planets Mars and Venus, a teleport spell can get you there. If one is a high enough level of wizard and can see the planet Venus or Mars in the sky, he can cast a teleport spell and go there. Teleporting to Venus is a little more risky than teleporting to Mars. The elves have optical telescopes for seeing these planets, but Venus is covered with a perpetual layer of clouds and 80% of its surface is covered with water, so a wizard is at risk of teleporting to an ocean surface if he goes there. Mars on the other hand is 80% dry land, it is a warlike place however, so anyone who teleports there had better go well armed!
Please tell me how this would change things? Some people who have teleport spells can go to those planets because they can see them in the sky. Elven astronomers can determine their distances, and so by that means they can teleport to them. People on those two planets can also teleport to Earth. Martians may have a bit of a problem dealing with Earth's gravity if they are not used to it, but their are spells that can overcome this problem.
Venusians have to get above their planet's cloud layer so they can see Earth in their night sky before they can teleport there, teleporting back to Venus is a bit of a problem for them because they can't see where they are going, but if they have been to a place on Venus before and can remember it, they can go to that place.
