Thanks for the chapter!
Item master doesn't just mean he can use any item, but use it in a way that maximizes item's ability to do something. For example, when he flung the gold coin to keep Gotte from killing the demon. While a gold coin is usually used for money it can be launched and inflict damage as a metal disc. His ability allowed him to launch the coin with the accuracy and power needed to do what he wanted it to do. After all, even an expert would have a hard time flicking a gold coin to do what Garrius did.
In the LN he talked with the demon king more and met his daughter too. Also, found out that the troops who were suppose to wait for his signal to attack the demon king's castle moved without him. Manga doesn't explain well but LN mentions that King knew Queen cheating on him and was using MC to clean house. Hence the crazed grin when Queen dragged away. Apparently the 1st Queen, Jared's mother, like current Queen was having an affair. King realized it when Prince grew up showing physical traits and talents that didn't inherit from him or queen. Hence the bumbling fool is just an act and has a talent and patience for long term revenge. So plan was to deliberately spoil Jered rotten and expose him as a fraud hero but Gotte's assassination attempt skewed the plan a bit. Unfortunately for Garrius the King used him as a disposable pawn for his revenge. Will have to read chapters coming up to see whether or not King has any more plans for Garrius or if content to do nothing if Garrius doesn't make a ruckus somewhere. That and if remains as king or not considering he doesn't seem to care about the position or country very much and only remained king to exact a very cold, cruel revenge on the Prince and Queen which he's achieved.
@yuu-oniichan Will have to read on to see what does with queen, but more likely grinning because has proof of her wrongdoing and can now execute or imprison her without having to worry about being criticized. That and spoiled rotten Prince was injured and revealed in public his pathetic cowardliness.