I'm I correct in believing that, with this wording, if a player uses the first activated ability while the Gift is attached to card X, and in response a player uses the second to attach the gift to card Y, then the card that ends up spliced is Y, and not X?

How does splicing work? It can't be like splice onto Arcane, because that needs to happen during casting.

Yes, I haven't yet figured a different keyword to use and I'm using splice as a stand-in. Anyway, the intended meaning is that you add the full rule text of the 'spliced' card on the target spell on the stack, and if doing that would require some extra targets you choose those targets as you do so (I should also probably find a way to word it so that it works within the rules)

So, to get that right, with Shadowmonger's Gift attached to a Concentrate each spell has effectively "multikicker " and "Draw three cards for each time this was kicked"?

The big issue is that this becomes available to your opponent first... if they have the nonpermanent card.

Yes, it is so. Though the idea was that any player may pay in response to the activation of the first ability to pull the rug under whomever tries to do something exceedingly powerful. Say the Gift is attached to Concentrate, player X casts some sorcery and pays 3 times to harmonize Concentrate trice to it; in response, player Y may pay once to move the Gift to something else of much minor effect - assuming there is such a thing in some graveyard -, and the card that would end up harmonized trice would be the second one instead of Concentrate

Feels like a mythic rare at least for the complexity layer it adds - it's basically a subgame around keeping mana untapped.

To me it looks like this design is entirely too much tangled up in its own mechanical implementation details. I would personally kinda just restart the whole design process with it by (re)conceptualizing what is the high-level conceptual intent, idea, and/or gameplay you are looking for this particular card to convey and accomplish.

Currently when I look at this my reaction is just basically that of confusion - and I'm almost irresistible forced to try to figure out what's the supposed intent by its designer rather than looking it as a card existing on its own.

Like, it looks to me that the mechanical weight of the card is solely on the enchantment rather than the creature, which begs the question why isn't it an enchantment to begin with? The creature hasn't a single ability that isn't directly about the generated enchantment - so does it really need to be a creature? It seems to me like that's just adding unnecessary complexity more than anything else.

The syntax of the wording is quite sketchy which in some part plays into it being hard to parse. I recommend checking Spellweaver Volute as an example of an Aura enchanting a card in a graveyard.

Tried to change it up a bit so that it no longer works by creating an enchantment to point to a card, and instead just imprints on the card and has an ability to change the card it is imprinting on.