Q:

One App for 7.1 and 7.0

Is there a possibility to add Mangofeatures (like secondary live-tiles) and still run the App on an 7.0 Phone without the Mango-Features? 
The Ebay App looks like it, but it could be that it has published 2 Versions, one for 7.0 and one for 7.1. 

A:

The answer is basically no.  You need to publish both a 7.0 and 7.1 version but they don't have to be separate apps as you can have 7.0 and 7.1 published versions of the same app. 
This was announced on the Windows Phone Developer Blog. 
[Edited to explicitly state that I am not advocating faking features]
If you still want to target 7.0 you have some options. 

Have separate 7.0 and 7.1 projects. 
Have one solution that shares code between two projects using file links, one targeting 7.0 and the other targeting 7.1 You should be able to share a lot of code and perhaps pull visuals out into user controls that could be shared. 
If you really want one app, though I don't think it's a good idea, there is a possible workaround using the answer from this question which might let you use secondary tiles.  

Along with option 3, you could probably also fake fast app switching by only handling tombstoning in certain situations with something like this (off the cuff, not tested):
bool isNewInstance = false;

public PageConstructor()
{
    isNewInstance = true;
}

override OnNavigatedTo()
{
    if (isNewInstance) 
    {
        // handle page set up as necessary
    }
    else
    {
        // handle tombstoning if necessary
    }

    isNewInstance = false;
}

