A. Last night I saw my aunt knocking out a gang of thugs with her bare hands within 15 seconds.
B. Would you still keep the intention of proposing to your girlfriend if you found out she was the current owner of a massage parlor?
Are these sentences natural to say in a conversation?
Thank you so much
"With her bare hands" is fine.
I can't say they sound like normal conversation, but I don't have any aunts trained in martial arts, nor do I know anyone whose fiancee own's a massage parlor.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
Sentence A looks fine to me.
I would change sentence B to read
Would you still propose to your girlfriend if you found out (that) she was the owner of a massage parlor?
In (under) 15 seconds sounds better to me that within 15 seconds.