The claim can be made 2 years after leaving Europe (European Union,Iceland, Liechtenstein, and Norway, Switzerland).
You will be refunded employee contribution towards social security(9.35% of Gross wage).
If you were working in Germany, 9.35% of your gross salary will be deducted towards social security irrespective of being a local employee or on deputation. Thus, Everyone who has worked in Germany is eligible to apply for social security refund.
Even if you have worked for 1 month, you have contributed towards social security. Thus, you can apply for the refund.
No. You can go back and start working as you did the first time.
You can get the social security refunded in any bank account worldwide.