German law is relevant to the release of this specific aircraft ONLY IF Iceland is party to a treaty that allows for German insolvency law to be enforced in Iceland (you have not cited any clear evidence that such is the case, and I do not know the answer for certain either).
No it is not a German peculiarity and it is quite general, however, the matter of jurisdiction is paramount. In the absence of a treaty, German insolvency law is irrelevant in Iceland. Yes, you are quite right - creditors would and DO pull this sort of maneouvre (it isn't "extortion", it is lawful seizure to enforce payment of a debt). When an insolvent company does not either by virtue of treaty or by filing parallel creditor protection in multiple jurisdictions to protect against such seizure it is the insolvent company that has failed to protect its assets properly.
This is nothing to do with gambling and losing, it is purely a matter of debt and collection of such debt when the debtor is insolvent and doing so in accordance with the laws which can be enforced in any particular jurisdiction.
If you want to argue the law, then argue law, which means cited legislation, treaties and regulations - principles alone won't win in a court of law.
Please re-read what I wrote.
1) airberlin, not Iceland, is bound by German insolvency law. And that means that as many snowploughs Isavia parks in front of an aircraft (that actually is not owned by airbelrin, but only wears its livery), airberlin as the debtor cannot make any payment to creditors for debt accrued before the insolvency filing. Making any such payment by an insolvent debtor is forbidden by German law, whatever tantrum Isavia throws. If Isavia does not like that element of German law, they should not do business with German companies, operate on a cash-only, advance payment basis, demand a monetary bond, ask for collaterals, whatever. If they do not, they are not different from anyone doing business with a company that may become insolvent or not. Like, for example, the employees.
2) A wholly different question - and a question of Icelandic law - is whether Isavia can impound a third-party asset that is not onwed, but simply used by the debtor. That may or may not be the case, but is nothing that concerns airberlin as they are merely the possessor, but not the owner of the impunded aircraft. Now, generally speaking, if you lease a car form a car dealer and don't pay insurance, or if you rent an appartment and don't pay electricity, from a legal point of view it would be very unusual if the company that you owe money could not only seize said car or appartment that you only possess and do not own, but also sell/auction it off in order to settle the debt of the possessor at the detriment of the owner. If this is the case under Icelandic law, it would be a rather peculiar feature as property rights would not be effectively protected (which usually is one of the cornerstones of a legal system as it is a prerequisite to attract investment). What you can usually do is to seize property the debtor has possession of because there is a rebuttable legal assumption that property possessed is also owned. But if possession and ownership are not with the same person, the owner usually has a legal remedy to get possession of the seized asset from the creditor.
3) Now, on a totally different page - a more practical one - is of course if the owner is happy to go through the whole legal process to reclaim possession of the asset he owns by wasting time in courts, paying lawyers etc. He might take a more practical approach and pay off the third-party debt in order to lease out the aicraft again instead of having it sit idle on the ground for months with the doubtful prospect of being able to claim damages from the debtor or creditor. Hence my mentioning of "extortion" - Isavia is probably playing this card. Which does not mean that they can lawfully not only impound third.party assets, but also convert them into cash at their pleasure.
4) What I do not know but others may be able to add is whether Isavia has indeed legally seized the aircraft pursuant to Icelandic law - or whether they have simply blocked the aircraft from moving in the hope of airberlin coughing up some dough to settle the debt. If so, this will not work as far as airberlin is concerned, see above 1)