If I recall it only appeared there after it was added (for old files in which it didn’t appear at that point). If you just straight up missed it during the section it appears then I don’t believe you can get it.
The move is still in the game (same as other Gym Leader moves that aren't available), but it is not longer accessible to the player (unless they already had it).
The Cacnea is a shadow Pokemon and you should have been able to catch it, even if they sprite is not correct (which it should have been correct), the game still tells you it is a shadow Pokemon when it is sent out.
Returning to Goldenwood Cave after beating Eli and Sharon (but before fighting Zetta) makes it so that when you get into a wild battle in the cave it is a double battle with Melia (despite her not actually being with you).
If I recall you can get to the left side of the door (it was fixed from beta). If you can’t from directly in the building you can from the Ralts area. Being able to surf there may or may not be intentional.