If you don't share blockchain history, then you don't automatically get some of the new coin by holding some of the previous coin. So there are no issues with exchanges and shorts.
Also if you do share history, it is much much harder to start, because you start off with the previous difficulty which is very hard to mine on. If you start a new altcoin with a clean history, the difficulty starts off very low.
Bitcoin Classic and Bitcoin XT have not actually forked the blockchain. So they are not separate coins yet.
> Also if you do share history, it is much much harder to start, because you start off with the previous difficulty which is very hard to mine on.
You need to fork the code (with your changed rules) anyways and you can adapt the difficulty then. Bitcoin Cash did exactly that. They lowered the difficulty to something like 10% of Bitcoin anticipating 10% of the hashing power. They only got 1% and there was a gap of 9 hours without any new block. They seem to be at ~1h per block now. [0]
No. XT and Classic were code forks, not blockchain forks. If certain runtime conditions had been met, they would have become blockchain forks, but those conditions were never met.
Also if you do share history, it is much much harder to start, because you start off with the previous difficulty which is very hard to mine on. If you start a new altcoin with a clean history, the difficulty starts off very low.
Bitcoin Classic and Bitcoin XT have not actually forked the blockchain. So they are not separate coins yet.