Hi Sandra,
Greetings from Austria ! - as I see you are in Germany 
Just a few comments related with your post:
1. The organic visibility dose indeed shows some drops that overlaps with both panda and Penguin. However you should also overlap the visibility with GA data in order to see what you rank for but you don't get any traffic - as this is a strong panda signal. Organic Visibility: http://screencast.com/t/0T0OdPBR As far as ranking distribution - it looks bad but I've seen much worst so it's not a lost case. ( http://screencast.com/t/z1wfh0vEjNbK )
2. With Panda you need to identify the pages with issues. If you do improve some pages (Made new, fresh, longer and much better texts for a few hundred pages) that's fine but it might just not do the trick. First you need to slice the site down and split it into chunks of type of pages (thin content, idle pages, low ctr & very high impressions, ratio of boiler plate vs content, downstream traffic etc). Then improve or delete or noindex those. After that consolidate as much as you can the rest. new landing pages are good - as it can change the threshold for Panda...
3. Over optimization is not a panda issue - you can focus if it make sense to improve overall and you might overlap with some panda signals but my personal opinion is that is not the main focus now.
4. Panda is "mainly" on page: by mainly I don't mean links I mean it's a bigger picture as it's about serps, it's about competitors. It's about being better then the other sites that are targeting the same terms - it's not about being perfect. Penguin on the other hand is about spam: on page and off page (un-natural links). If you do clean your link profile make sure you clean it base don some good metrics. Also include the links you remove in the disavow file. The disavow file will be processed only when Google will roll out Penguin again. Until then don't expect any changes (unless you also have a manual action and when that is processed then the disavow will also be processed.
I would say Panda is easy to fix in your case - as it anyway doesn’t look like a "kill shoot". Penguin on the other hand will probably be harder but since soon we'll see another update you should hurry and hope you get on this train as otherwise you might need to wait for months and months....
LG
***I bet some US folks would think that those (LG) are my initials 