Resolving of the DB-properties is done using a org.springframework.beans.factory.config.PropertyPlaceholderConfigurer. Make sure there is only ONE db.properties on the classpath and make sure that contains the values you need.
Explorer caches users when it boots, so modifying the database while explorer runs, will not update the cache. Rather, modify all user-related stuff from within the "manage" tab of explorer, this passed though the cache and updates accordingly.
Alternatively, you can edit the activiti-standalone-context.xml and replace the {jdbc.username} placeholders by the actual values or use another way to define a datasource (eg. JNDI, …).