Mix of things here.

1. Yes, he tell Vin she could leave. She also had nowhere else to go and was being hunted by multiple government agencies. Those agencies killed basically all the people she knew before meeting Kelsier. Stay with me or face probable death alone isn't a meaningful choice.

He could have, for example, offered her the option to hide with the Skaa rebellion in relative safety (as far as he knew at the time, they weren't going to act without him), or he could have gotten her a job working for his Kandra nobleman in relative safety below the radar. Either would have been a safer option that wouldn't have put her in situations where she had to kill or be killed.

1. Vin was trained to blend with noble society at fancy-dress balls that are intended to encourage match-making among the nobility. A honey pot is exactly what she was trained to be, she was supposed to extract information from young noblemen who intended to impress her by spilling house secrets. Sazed and Kelsier wanted to keep her away from Elend specifically because she risked her usefulness by being attached to a single boy, and they feared her potentially dividing her loyalty.

Sazed clearly did care about her beyond her usefulness, but Kelsier as a cognitive shadow explicitly boasted that he killed The Lord Ruler, and backed it up by saying that he "forged the knife that killed him". Not that he taught a student who killed TLR, that he forged the knife. He saw her as his knife and used her as such. He only came to see her as a person as time went on.

1. Yeah, I'd also have had a very hard time passing up having another Mistborn on my side. But she was still a child who was exploited.

If our protagonist was working in an Oliver Twist victorian-era factory we'd condemn the factory owner for exploiting child labor. There's a double-standard because we sympathize with Kelsier's goals and the outcome, but by all rights we should condemn Kelsier even more harshly; he exploited child labor for spying and murder.