I'm sure 99% SCs will use 1:1 conversion rate and no new SCBTC will be generated ... you will still have your bitcoins, but few will be locked in "fast transfer" sidechain ... you will have some locked in "exchage" sidechain ... and some in your private business sidechains (not public sidechains)
We can give ourselves 50% of the ether initially and let the next 50% come from locked coins. A pre side mine if you like. As soon as a decent number of coins are locked into our chain we can just cash out.
You certainly could. You could unlock (get on the bitcoin chain) exactly as many BTC as were locked regardless of how many ether you created. So you could leave a bunch of bagholders on the sidechain.
How is this any different from Mt Gox, Moolah, altcoin pump and dump, etc?
Sidechains don't magically make people honest.
But one additional point: the "bagholders" on the sidechain now have coins whose value would float relative to bitcoin (there is no backing BTC). So what they are left with is better than Gox or Moolah -- they are left with a functioning altcoin that presumably has some utility to them or they would never have moved BTC there in the first place. A similar result could happen if someone 51%ed the chain and unlocked the backing BTC.