Mock role-play with both parties at the same time (remote or in the same building). Schedule an hour session, pay for their time. This is the fastest method.

In your question, you mention "often takes several months". I would assume there is a scenario that doesn't take several months. I would focus on testing only those shorter scenarios if possible to reduce the timeframe from other types of lengthy encounters with your service.

It sounds like you already have a good idea of how your users will interact with the service. The best idea is to figure out (1) what tasks you want to test; (2) create realistic scenarios that you can have users "imagine" they are in the middle of and (3) design the prototype to fit those tasks and scenarios. If you try and prototype the whole thing (or most digital services) you'll no longer be doing it rapidly!