Could 'ten człowiek' also be accepted?
Yes, of course. Added.
Why not mężczyzna?
"mężczyzna" is exactly the main answer.
Ten pan jest niebezpieczny. ? too polite for the subject??
Well, I personally think that using "pan" as a noun often sounds a bit like you're talking to a child. But anyway, it makes sense, added.
Maybe in my case it's more like talking "like" a child ;-)