Can I have a hint where to find it? I think there's some gold chests on the starter island

I apologize for that. If you leave the area to the east back the way you came this issue should fix itself. 

OK, I got the item. Trouble is, despite the game saying Mercury learned the spell, when I try to enter the cave it just says the cave is flooded. The Aqua Lung Spell is in my key items, but not in Mercury's spell list. Is this another glitch, or do I have something else I need to do?

Got it. What tripped me up was that the three labyrinths had different amounts of orbs. That said, there's something I feel I should inform you of; after I got the three labyrinths completed and rescued Oriel, when I returned to the library and freed the elemental beasts, then entered the underground door (no need for any new keycards, the door just opened.) 

After entering, the scene with Kamran and Mercury repeated itself. After the scene ended, I beat the high priest and entered the knife chamber (didn't grab the knife, just took the two treasure chests and left), after I tried to leave, the cutscene where Melody shouts at Kamran to tell him the fort is under attack also repeats, and from there I went to the fort, and Dominus was back in the cell. 

After talking to him, I was automatically sent to Red Wall Village, and Mercury left my party (probably because he leaves when the scene is originally shown.) Anyway, I reloaded my save from the knife chamber, and after the Melody scene forcibly removed me from the library I just proceeded to the next area, where the game continued on as normal.

Alright thanks, sorry for panicking like that. There's a lot of things I'm trying to keep track of in the game even if I took notes at my leisure.

oh okay. good to know.

Yes, it is V2.    Where to get the bomb？

NO

Do the crystal tablets have any effect on the story?

And by first two do you mean the ones before you add Mercury?

Has that placeholder been replaced in the latest update?

Oh boy... I'll take your word for it then. I was worried about missing something important but this helps a lot thank you.

I think I know what they meant. When you first see the King, the player is tasked with going east to find Mercury but you can skip that whole section without even going there once by just going into the underground part of Glittergold Town past where Melody stops you your first time there and past that you can just trigger the "shard" fight with your whole party. I was confused but loaded my save and did the east part to find Mercury first and then understood what was going on. I suggest adding something to stop the player from accessing that underground part after the  Melody interaction until after doing the east area.

EDIT: Also, the fight after breaking out of the the cell was much easier with all party members (which obviously shouldn't haven been there if you do the east section first). Doing the fight correctly (alone) turned into spamming healing items because most of my skills were unusable due to having no weapons equipped for it which shouldn't be the case because the player supposedly had the black dagger because the king couldn't take it but I was unable to use any of the dagger skills in the fight. It was a very tedious fight. Lucky Stab would've been great there. : ) 

I had finish the event at the little village,and get the item from the ruin,so what should I do next step to through here?I talk with every NPC,but no new event happen.

Oh. Ok thanks. I was just a bit confused but it makes sense now. Thank you.

Ack, I didn't think straight because I panicked like a noob. Thank you for the help.