I will start with ontology. The main questions that should be answered here are: what does the existence of the world of Heroes include?

If you are talking about cosmology, then the structure of the Universe in Heroes is no different from the real one. It consists of many metagalaxies (which in turn include billions of galaxies), nebulae, and so on. The action of all games in the Might and Magic and Heroes of Might and Magic series takes place, apparently, in the Milky Way galaxy (which is where Earth is located).

Does a higher power exist that created this world?

It exists (I am talking about the planet of Heroes 1-3 and Might and Magic 6-8). More precisely, not created, but brought intelligent (and almost all other) life into it, changed its ecosystem and biosphere to its current state. This is a civilization with enormous technological development, called the Ancients.

How do Good and Evil relate to each other in the world of Heroes?

As you already said, there is no clear division between Good and Evil in the world of Heroes. But if we consider it from the perspective of the Ancients mentioned earlier, then Evil is:
devils
demons
everything else that harms their experiments,
Good:
everything else that does not harm the Ancients' experiments.

If we look at Heroes 3, then the Castle and Rampart are considered good factions, while the Necropolis and Inferno are considered evil.

Here is my classification of the alignments of the factions in the third Heroes game according to D&D (based on the events of Might and Magic 7 and the campaigns of Heroes 3):
Castle - Lawful Good, Chaotic Good, True Neutral
Rampart - Lawful Good, Neutral Good
Tower - Lawful Neutral, Chaotic Good, True Neutral
Dungeon - Neutral Evil, Lawful Evil, Chaotic Evil
Necropolis - Neutral Evil
Inferno - Lawful Evil, Neutral Evil
Stronghold - Chaotic Neutral, Lawful Good, Chaotic Evil
Fortress - True Neutral, Neutral Good, Lawful Good
Conflux - True Neutral, Lawful Good, Chaotic Evil