No sure how looks your alert condition so I can't tell. But suppress alerts for unreachable status is ordinary operation. That's actually reason why we have unreachable status and possibility to define dependencies. That you are not bothered by alerts which are just consequences of events on parent node.
"Summary email or macro which can tell you which nodes are unreachable because this node is down ... this is not possible right ?"
- no it's not possible - but you can place into your alert message link to "web view" which will show you all dependent nodes.