I just read the thread, because I'm in exactly the same situation now - need to connect 3com 5500 with EX-VC via LACP-Aggregated links.
First, I would like to ask you to post / mail me the 3com-config as well as I don't know anything about 3coms.
If anything changed in the EX-config since your last post regarding the lacp-stuff, please post the ex config again as well.
Second, I'd like to know, if you have sorted out your switchover-problem.
Thanks in advance