Is it possible to abuse snowflake to do e.g. illegal file sharing or other illegal (illegal in Germany) things and my IP is exposed?
With snowflake your IP is not listed. You don’t need to worry.
By running a Snowflake proxy you allow others access the Tor network. You can’t know what they are doing with it.
As said on the store page of the extension, your IP is not exposed. You simply pass bytes between the client and the hard-coded bridges run by The Tor Project.
I’d say running a Snowflake proxy is legally equivalent to running a non-exit Tor relay. See Tor Project | The Legal FAQ for Tor Relay Operators.