I think we are conflating separate things here. Spells, Hidden Enchantments, and Revealed Enchantments are all separate despite being represented by the same physical object; a card.
Spells, including Enchantment Spells, have lots of characteristics, such as traits and subtypes, that affect all sorts of things. They affect spellbook building as well as the abilities for different objects to prepare and cast the spell. They affect which targets are legal and the cost of the spell.
Hidden Enchantments have no affect on game play, unless otherwise specified. This means that they have no name, traits, types, subtypes, text, or other characteristics. Consider the front of the card to not even exist. While it's face down, it's a completely different object.
Revealed Enchantments again have lots of text and characteristics that tell you how they affect the game while they are in play.
Now that this difference is established, let's discuss some interactions. Casting a duplicate enchantment when the first is hidden is covered by an "otherwise specified" ruling that says known Hidden Enchantments can prevent further castings of the same card. This does not grant the Hidden Enchantment any affects on game play beyond this, so the Enchantment still does not have a name or any other characteristics. It's simply recognized as a known duplicate when determining legal targets for the new spell being cast.
Harshforge Monolith is another "otherwise specified" deal, where the Monolith specifies that Hidden Enchantments now do have a trait, the Upkeep +1 trait, while they are within it's range. The enchantment still is not a mind spell while hidden though, so it does not get the discount from Psi-Orb.
The Mind Shield question by Exxid is an interesting one. Technically I think you could reveal it during the casting of an enchantment spell. However, you would just be guessing as to whether or not the spell was a legal target. I do not think the rules currently cover how to handle this situation. If you are wrong, then you have to discard Mind Shield without effect. If you are right, their spell is countered. But how do you determine whether you're right or wrong? Do they just tell you without revealing or do they have to reveal their spell to prove it? If they reveal, that gives Mind Shield a powerful new capacity that I don't think it was designed for.
I would personally play it as they just tell you without revealing. After all, there's already a lot of inherent trust with enchantments, such as the duplicate rules and ensuring specific objects cast the correct types of enchantments, and none of it can be proven until they are later revealed.