The architecture of jBPM BPEL is simple. It uses jPBM for executing the process graph, J2EE 1.4 web services for invoking/providing functionality and JMS to dispatch incoming/outgoing messages internally.
1. Given the above, the minimal server configuration won't do. You need the JMS and WS components (and their dependencies, of course). The "default" and "all" server configurations work.
2. BPEL does not require all the functionality the core jBPM distribution offers. Hence it inherits part, but not all dependencies. The actual dependencies are all included in subdirectory lib.