And how do I store character from actual campaign with it's lvl and items so I could use it in new one?
you can't - the soveregn at the start is saved, but level and items are not (otherwise it would be too difficult to balance with the other characters)
re the other questions
2. I don't think anything else would be necessary as that would confirm the purchase date.
3. Not yet - you would need to rework the graphics for every single item. No one has done that yet.
4. In what sense don't work - you can see some achievements but can't do them? - they normally only wiork when you start the game through impulse in any case.
5. You can't save the updates, but if you reinstall impule can patch to the latest version. There was some talk about sending ot CD's with uopdates, but I didn't follow that too closely.