Hello Alfie,
I've read through your post and I think that jBPM is suitable for your scenarios.
Several notes:
- manual tasks in BPMN2 are more for documenting purposes of activity which is not executed in the engine, than real usage
- you can use user tasks for user interaction
- you can use own defined service tasks for any synchronous or asynchronous interactions with systems
- as you say arbitrary flows and logic can be defined using gateways
- BPMN2 and jBPM is able to map process variables to task inputs and task outputs to process variables
You can decrease predisposition to errors for complex processes in multiple ways. For example you can use subprocesses to achieve better abstraction.
Also you can always write unit tests for your processes to automatically check their expected behavior.
My 2 cents.