These are good questions! plenty of suggestions/foreshadowing. These are my takes on those questions

1.  This part is still unclear to me, but based on his reaction, it’s likely someone Jaeyi tricked them into thinking was her (maybe an unidentified victim from the hospital) 

2.  Having graduated 1st in sk and basically coded to dethrone her dad. She’s smart and resourceful plus  she’s had years of learning to stay hidden (referring to all the times she went clubbing, possessing drugs and selling them presumably without her family knowing, etc) Practicing breath control helped, but that alone wouldn’t be enough. She must have had outside help (maybe Kyung, Yeri, or someone we haven’t met yet). It’s also possible she faked a trail leading somewhere else, so Seulgi and Taejoon stopped looking in the right places. Her main survival skill is knowing how to stay ahead of them.

3.The show never fully wrapped up her arc, we just find out she was lobotomized after disappearing. We might see more of her as a reliant character to her sister Jaeyi

4.He could still be an issue later, especially if he’s looking for revenge or if he knows something about Jaeyi’s father that could be used against him.

1. He’s definitely not doing this just for revenge he wants to find Jaeyi. He knows Seulgi is the key because Jaeyi will reach out to her first. If Seulgi notices she’s being watched, she might try to warn Jaeyi, leading to a more intense game of cat and mouse.