Why would nobody accept those BTC?
Ok, "nobody" is an overstatement. However, say 1% of businesses love the new chain and decide to switch over. Now you can spend your coins mined on the fake chain somewhere. Not in very many places though, and you will have to go through all the massive adoption pains that bitcoin currently suffers from. So perhaps you'll get a few extra coins for your work (then again, you won't because
you control 99x the hash power of the original chain), but they will be worth, as a whole, far, far less as not many people will want them. Not to mention everybody with existing coins will be able to double spend them all on the new network and basically bring the value down to nothing.
The new block chain honors all BTCs mined in the old block chain. However, it offers more attractive conditions to every miner which adopts the new block chain. Therefore there is a clear incentive to switch to the new block chain.
No, there isn't, and I'm not going to keep rehashing this.
the old chain will lose 99% of its hash power and will be V-E-R-Y slow.
This can be an attack in and of itself. Probably only likely if someone finds a vulnerability in SHA256 and keeps it to themself. In which case, bitcoin devs can fork it themselves to use a new hash algorithm.