The younger population of South Korea is generally opposed to reunification: http://www.cbc.ca/news/world/pyeongchang-olympics-korea-unif...
Ending Germany's limited sovereignty did not require reunification. It required (a) the Four Powers renouncing their residual authority over Germany, as they did in 1990's 4+2 Treaty (b) an agreement about what to do with West Berlin. The obvious options for removing occupying power control of West Berlin without reunification would have been (i) West Berlin becomes an exclave of West Germany; (ii) West Berlin is integrated into East Germany; (iii) West Berlin becomes independent as a third German state.
Why would the UK have even pretended to want to stop it? Weren't their foreign policies, at least towards communism, heavily aligned with the US?
Also noteworthy, the Soviet Union dissolved shortly after, at the end of 1991.
Whereas Germans, East and West alike, had largely thought of themselves as Germans in varying senses since Bismark (and both the DDR and BRD claimed to be legitimate successors to the same historical state), the peoples who lived (and live) in Northern Ireland didn't all think of themselves in the same way. Some thought of themselves as British, some as Irish, and many as some mix of both. A big reason why historians mark the Good Friday Agreement as the end of The Troubles is that it recognized the right of each person in Northern Ireland to identify with the nationality they chose (self-determination), in a way that both the British and Irish governments cooperated to recognize in administrative affairs where nationality mattered.
There wasn't a separate ethnoreligious East German identity. There were (and are) conflicting identities in Northern Ireland with ethnic, religious, national, and historical dimensions. The two situations on-the-ground were extremely different.
The opposition of the rest of Europe and Russia has to be put in context. Russia would not have been able to support the East German economy nor would it have been able to afford to maintain a pointless troop presence that would no longer have local support. The rest of europe might have been opposed because of historical reasons having to do with security but not allowing unification would have created a massive roadblock for the european economy. Short of reunification, there was not an acceptable legal regime to end Germany's limited sovereignty(which the rest of europe supported).
Given that Germany strongly supported reunification and the US was in support for obvious geopolitical reasons there was no way for any other power to stop the process. Russia could either be paid to leave or get nothing and spend money it doesn't have to maintain the troop presence to boot. Poor relations with Germany would have had a catastrophic impact on the Russian economy. The UK was never really serious about stopping reunification and barely tried to bluff. Similar reasons apply to France which could not oppose reunification and support European integration.
For all these reasons I find the the focus on contingency to be rather overdrawn. It was not inevitable in the sense that it had to happen(you can come up scenarios where the soviet union continues into the 21st century) but given the collapse of the Warsaw Pact it's hard to understand how unification could have been ultimately prevented.
You can see a similar dynamic taking place in South Korea where unification is broadly supported by the younger population in the face of titanic security and economic concerns. German unification has been supported for hundreds of years for comparable reasons.