Yup will do for game breaking ones.
No, it just gets you a bit of different lore for each one, there is nothing you can miss.
Looked it up and yes the gardining quest was the one in the western plains. It's in this building on the western plains. Since you have gotten the quest the building should be in that version as well.
The casino is optional to the game. It mainly serves to be able to get ultra rare and limited items. I think one plantable seed and a fairy seed can be also bought one time in the casino but they aren't needed for any progression. The only time you need to visit it is to buy the Fuwa Plush for a side quest. And in a later version there is a quest related to the gacha as well.
But you never need to play each casino game, you always could only play one or even just buy the tokens for money.