The cryptography is exactly the same as Bitcoin', with the one exception of bitshares having a slightly stricter definition of canonicality.

As explained by Xeroc, this is the reason for that:

Once we ensured that the signature is canonical, we derive the so called recovery paramter. It simplifies the verification of the signature as it links the signature to a single unique public key. Without this parameter, the back-end would need to test for multiple public keys instead of just one. So we derive this parameter, add 4 and 27 to stay compatible with other protocols and have now obtained our signature.

See this post for more details:

https://steemit.com/steem/@xeroc/steem-transaction-signing-in-a-nutshell

Check your api config, Settings -> Access. Make sure you're not connected to the testnet.

Even with the backup you still need the password. Keep trying, there's no limit to have many attempts you can use.

You go to the settings and then restore using the private key directly.

It's a decentralized exchange which means noone else has your password or private keys. It clearly says so when you create an a count, and it stresses the importance of storing your password and backup since noone can help you recover the account if you lose those.

You have to remember that unlike Poloniex bitshares is a blockchain based exchange which imposes some limitations on what is feasible in terms of performance.

Most if not all the changes you're talking about here would require a hard fork, but I can't judge if they're even technically possible.

One idea that I like is being able to close your position by using the collateral you've already set aside, I suspect the reason for that is the lack or market orders. However, that could be worked around by an operation that placed a limit order with a high enough price that it would just walk the orderbook. It would need to be a fill-or-kill order so that it fails of it can't get filled completely.

Not sure it should work written like that, you should put the ?r=x at the very end of the URL.