More like it's possible the dungeons he's went to were tapped out. Most manga, this one included, implies dungeons are used more for specific things not found outside.
Keep in mind, that while slaves, they are feed and provided for to some extent. If he just summons more than they can provide for, they'll just waste away and die a possibly worse death. If he can make the conditions better, then the slaves can still be saved AND not be thrust into a situation...
The elf girl said she can use the spirits of nature. The guild lady said from the start supplies would be sent regularly to help development; that's kind of development 101, to send supplies to the developing location. That'd be like saying it's convenient California gets so much water when...
If we use Christian logic, then the class is the roll you're meant to play; not the one you WILL play. Due to free will, you won't always choose it. Great and wise men started off good, only to fall later in life in the Bible. Seeing as how the [Paladin] class is here, they are likely true...
Remember, until the European nations and the Americas stopped it, slavery was a world wide thing until about 200ish years ago. If this takes place in a world like that, it'd only be natural that slavery would be there; same with other manga. The fact you seemingly have the closet thing to an...
Maybe if they were friends for a while, but half the girls started off as bullies and had almost no time outside of school with him. The fmc was also the one that went above and beyond, thus the thank you gift. Guys tend to give practical gifts, and this was not only a house invite, but also a...
I'm not sure he's leading them. Remember that until recently, ALL the girls besides the fmc were not genuine in their initial meeting or relationship. They either teased, bullied, blackmailed, or insulted him. It's quite understandable that he would keep this in mind and just start to trust them...