Hi Matthias,
You should not modify any javascript files included with the editors. That file is not even used, that is the file that would be used if we were not overriding it to use our service url for styles.
If you want it to only use your custom styles you should add this to your user.config
<add key="AddSystemStyleTemplatesAboveSiteTemplates" value="false" />
<add key="AddSystemStyleTemplatesBelowSiteTemplates" value="false" />
for more info see Creating Content Style Templates.
Regarding the special characters errors, in my testing it works fine in MySql.
Does the problem occur in all features for you or only in specific content features?
Does using a different web browser make a difference?
Best,
Joe