You can provide a screenshot of how you configure the item metadata in MainUI.
If you configure your item as Blind, you should see the below control page. However, I can’t confirm if this is supported with in the Alexa app in your language. Maybe someone using the app in German can confirm. If you directly get the settings page when selecting a device, it usually means it is not supported.
As far as Alexa groups, semantic requests, such as open, close, lower or raise, are not supported at the group level. You can resolve this by exposing an OH group as a single endpoint.
