My question is very simple: where are all the BTCs?
If you were to look at Mt. Gox's cold wallet(s), I would bet that you would be very surprised to learn how many could be found there.
Having a BitcoinFund hedge fund acquiring and holding more will help cause more to be transferred out of Mt. Gox perhaps, but that's still about the same -- large amounts of funds ending up under the physical control of a small number of entities.
So where are all the other BTCs? Are they stored somewhere?
Bitcoins held by those running the Bitcoin-Qt/bitcoind client are stored locally. Bitcoins held by those storing them to paper wallets are local as well.
The reason you aren't seeing them traded is because they were acquiring for speculative purposes. They aren't being traded because the speculators aren't selling. (Well some are, certainly, but many more are not.)
And the most important question that worry me much: what if the holders of all this BTC decide to move them to exchangers and to sell them?
Well, the market at current levels would probably be decimated if a half million coins were to be dumped over a period of just a couple days. The carnage would be even worse if let's say a couple million coins of supply were to overrun the demand.
Will we easily reach the bottom in that case?
It all depends on why the selling is occurring. For instance, let's say a rogue programmer somehow slipped in a keylogger+file transfer utility into the most recent Microsoft critical update. On April 1st, 2013 at 00:00:00 UTC it is set to execute, and will upload to the cloud every wallet.dat and keystroke log it was able to capture. By 1 am the attacker has started dumping large amounts of coins and the exchange rate drops. Then users start reporting that their wallets are showing transactions they didn't make. By 2am it is apparent there is a widespread exploit and possibly millions of coins are now in the hands of thieves, and the selloff accelerates.
Possible? You betcha!
Likely? We'll probably first see something like this from some lesser known third-party software, but since wallet.dat + keystroke log = "free money" (in the eyes of the attacker), it wouldn't surprise me if it were to occur.
Everyone holding bitcoins are vulnerable to the loss in confidence and the resulting selloff following such an event. Whether that means the exchange rate drops to $2 or $12 or $28 nobody knows. But absent some armageddon like that, any sharp drop will probably be met with heaving buying from those looking to enter a position (or buy back) by picking up some "cheap" coins.