[FabHK]

So, first of all, the net shares outstanding are still 100. All the extra, whatever, 200 shorts are balanced out by 200 extra longs, and that creates obligations between them, which must be managed as usual (collateral, margin calls, risk limits, ...)

To short a share, you must borrow and sell it.

If you buy a call, you're long. You could write a call and then you'd have short exposure, indeed, but on the wrong side - you lose on the way up, while you want to win on the way down. So, you could buy a put - that makes you short, winning on the way down. However, now the entity that wrote the put is long, and will generally cover their exposure by - shorting. No magic bullet there.