How to rollback last executed sql script fired as sysdba
The default tablespace of the SYS (sysdba) user is typically SYSTEM. From that you can figure out the .dbf file being used, although I'm not sure how this will help.
Maybe. Does your database run using flashback? Also, manually backing out changes (or write a script to undo what you did) might be possible.
Maybe.  What changed?
Yes.
Be more careful.  Ask this question on dba.stackexchange.com.


EDIT:

I would add that running scripts as the SYSDBA user should be VERY carefully evaluated as to necessity, and probably never used for application purposes.  As you can see, the SYSDBA user has essentially unlimited power over your database.