Thats very odd. Is your computer older by chance? If it isn't, when you open RPG Maker, try allocate more RAM to it. It might just not have enough to run properly.
Yeah, I know about that bug. It's set to check the wrong item. If you have 5 pig in a cases (It's alot I know) but it will continue that questline.
Also knew about that one. I just made that event poorly to be honest with you. That's why it does weird stuff.
There's only one place currently. It's the cave in the Icycull Mountains. Although, if your above Size 8, you can just smash through the wall. (Or if your below size 2, you can just walk in normally). I've been meaning to add additional uses for it. Just not sure what.... :D
The chasm in the sewers in only usable if you LOSE to the queen the first time. It just takes you into the castle library (but if you beat her, you can just walk in through the front door). There's nothing beyond that point honestly. The reason it's like that, is cause you get kicked out of the castle and they change the locks (so your key doesn't work anymore)