if you want a spoiler, they kidnapped it from Ms. Ling's group (and if I remember it correctly from the novel, its Ms. ling's child). So, from the early on they trained it for them to have the upper hand
Yes Fugui had to kill Qingtong. Then he died as well.
both of them did not really died but they sacrifice. at the end, there's an open ending where both of them transform into butterfly and a spider then be one with the nature
in ep1 min 2 they give a hint. They only wrongly assume he (ML) is their ancestor...But I had the same thoughts…
I also assume that the ML is not really their direct ancestor, the ML's guard/bestfriend maybe. Cause if the FL is the girl in the picture (they married and all) and given from the clips of the song intro she will come back to the future, so there's no direct descendant made. Maybe the ML give his properties to his loyal best friend and that's why the Shens' family have the old house mansion.