The $2T cited in the book does not refer to the direct costs paid to Russia but to the total amount spent on reunification which came out to something like $100 billion per year for twenty years.

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.