Doing this kind of operation is risky, because Bitcoin's volatility can be higher than the price of BCC. Lets say you buy more Bitcoins at $2800, it drops to $2500, and you get BCC at $150; so you lost 10% at BTC, but gained 5% at BCC, which is a total -5% change. It would be more profitable to simply trade BTC without taking BCC into account, if you can predict price shifts really well.

You left out one major thing, though, and that is that it costs money to borrow bitcoins to margin sell them (the interest rate). In the days before the BCC split these rates may be very high because people aren't loaning out their bitcoin since they want to hold on to it to receive BCC.

That is because when you short a bitcoin NOW, you are actually shorting a bitcoin AND a BCC after 1 august.  Because the bitcoin you are shorting now has split in two.  If, of course, your exchange is somewhat dummy, you may get away with just giving then back a "bitcoin-after" and put the other half in your pocket, yes.

You see, that's the other side of looking at:

"if I have pre-1-august bitcoins on an exchange, will they give me my post-1-august bitcoins AND my BCC coins after 1 august ?"

If you've shorted a pre-1-august bitcoin, that became a short of a post-1-august bitcoin and a short of a bcc coin.  Unless you can do what some exchanges tried to do with the ethereum split: keep the other half of the spit coin in their pockets (coinbase tried that for instance).  Yes, you could try to keep the other half of the split coin you shorted, in your pocket, true.  If they aren't smart enough at the exchange (because they want to keep their customers' BCC for instance and don't see how they could tell you you've shorted BCC if they pretend it doesn't exist) you may get away with that.

But the rule is: a pre-1-august bitcoin = a post-1-august bitcoin AND a BCC.

In any financial operation that bridges 1 august.

You can understand this if you take, for sake of argument, the extreme vision that it is bitcoin that falls spectacularly in price, and that its market cap is essentially overtaken by BCC (I don't think it will, although it should: BCC doesn't have the original bitcoin name, and all bitcoin has, is a name, nothing else: it is that name that differentiates this old clunky crypto currency from all "alt coins" that work much better, but don't have the brand name).

Suppose that bitcoin drops to $100,- and that BCC is now at $2500,-.  Redo your story.  The exchange will claim that what you shorted was a combination of post-1-august bitcoin AND BCC.  And they would be right.  They could even say that they drop original bitcoin from their listings, and that for them, essentially, what was bitcoin before, is BCC now.  Then your "proof" proves that bitcoin's value after the split cannot be more than 0.5% or so.

Here's the thing, I expect shitmaincoin to be dumped hard after Aug 1st sub $10. Afterwards, the Bitmain cartel will prop the price up to around 10% of BTC and try to keep it above that going into November as a threat against not activating the 2mb fork. In any event, it will be interesting to see how this all plays out.

You are assuming that there wont be enough demand to support a higher price. Maybe, or maybe not.

Considering the number of powerful investors and mining pools supporting BCC, I really doubt whether the price will be that low. The exchange rates are going to be very volatile, but my guess is that it can range from anywhere between 1% and 10% of the Bitcoin price.