It looks more like the Leviathan rather a turtle but oh well xD
Stalling out the rain/changing the weather to Sand/Sun might help if you can. Once you do, slam it with Super-Effective moves.
Also, Grass Knot comes in handy, which is a TM you can find.
You pretty much have to get the Key again.
However, instead of going for that pathway at first, try interacting with the Statute that Narcissa was guarding before. The rest will be up to you.
Well, there is stuff to do such as looking for the new mons implemented, searching around for hidden items with your Itemfinder, breeding, etc. It all depends on ya mate.
And to give you a hint on how to open it, I'll leave it in the spoiler below.
Correct. Well, I'm not sure about the Destiny Knot since we don't even have access to it, but the Everstone does work for passing down the nature of the holder.
Yea, I do agree. It is only due to that Spoiler Lock Thread by Chubb I was saying that otherwise it would be fine.
Speaking of which, I should have did the same.. I'll edit my own.
I don't recall there being a shop for them. I think there was a person that gave you a bunch of them. Also, you could have just found some along your journey so yea.
Moon Key? I have no clue unfortunately :T Can't help you with that.
As for that gate, I think we can but I haven't tried to look for the item that the girl was saying to look for.