Are shorting and longing symmetric? IE, do they influence the market in the same way? They seem much more complicated than straightforward stock purchase and fractional ownership.
Yes, they're symmetric. Shorting is logistically a bit more complex than going long, but that's not an argument against it. After all, buying stocks is itself more complicated than buying land, and buying land is more complicated than renting, etc.
Don't know enough to say but I would guess so.
In the generic case:
Longing == buying a share
Shorting == selling a share
I'd add to that and say shorting is typically selling a share that you don't own. Which means you need to borrow it from a broker for a fee. There's a securities lending market which revolves around borrowing and lending shares for these reasons.