It's not like a call center putting you on hold. The clients that have requested only microservice A get now a response immediately instead of timeout.
About the fault-tolerance, that's right, but you still prefer that if fault happens in one microservice it won't affect unrelated services.
Hi. You don't have to change the library in order to make it work through fibers. You have to wrap it. In case it has efficient implementation of the async-api you have to implement the fiber-synchronous using the asynchronous api. If it hasn't you have to wrap it with threadpool. You can take a look in the implementation of the JDBC wrapper here: https://github.com/puniverse/comsat/tree/master/comsat-jdbc