Thanks a lot for the comments, larimar. I hope my simplified example model conveyed what it was supposed to convey.
1) I generally use UML for modeling systems processes prior to their development; but in this case, the system exists so I tried to apply BPMN. Additionally, the BPMN Quick Reference Guide (QRG) on this site, as well as many other examples that I've seen on the Web from reputable BPMers, use BPMN to document general systems processes. I initially modeled this process using a Sequence diagram, which seemed to be the best UML modeling tool to use. The Activity diagram, in my opinion, is lacking, too simplistic, and doesn't, in this case, at least, have what I need to convey the truth about the process. Plus our stakeholders, as well as developers and management, are really starting to see the benefits of BPMN for accurate modeling.
2) BPMN specifies that each pool contains separate processes. Participant B, in my real-world model, is a third-party vendor, not part of our organization. Our means of communication are HTML post-backs (i.e. messages). Again, the BPMN QRG on this site shows messages transferring from the Travel Agency to the Funding Institution: This is very similar to my process. The fact is that the Participant A pool process is finished when it attempts to transmit the data to Participant B. If it can't transmit the data, then the Participant A process is still complete, only the message is not sent to Participant B to trigger the event necessary for the Participant B process to continue. There is no parallel processing occurring here, by the way; every task is sequential and depends on receiving those post-back messages in order to proceed.
3) Great point, and I had thought about making that a collapsed looping subprocess. I still might do that...
4) Technically, the process for Participant A is complete, whether it times-out or not. A trigger is required for the Participant B process to proceed.
This is a rather challenging prospect, in my opinion, and I'm sure I'm not the first one to have to deal with it. Sure, I could put both participants in separate lanes in the same pool to make things simpler, but really don't think that's conveying accurately what's occurring.
Again, thanks for your comments.
vinny