On August 4, Clarke from the MobileMe Support Specialist Team made contact via email to ask whether I was still interested in resolving this case. Although pleasantly surprised by the renewal of interest on the part of Apple initially I ignored the mail fearful of another waste of time with an agent who knows nothing. However, when Clarke again contacted me on August 19th I thought I'd give him marks for persistence and respond. MobileMe Support must have more bandwidth now - maybe because everyone has canceled already? :-)
Responding to Clarke turned out to be a good move as he reactivated my expired MobileMe account in order to continue to debug the problem. He also gave me a program called CaptureData which grabs all sorts of data from your machines (including packet traces) into a dmg which you send to Apple. Given those carrots I've sent him 7.6Mb of data to chew on! Hope he finds something in there.
Since the initial problems earlier this year things have moved on a bit. We have moved from Leopard 10.5.6 to 10.5.8, with Snow Leopard due out tomorrow! The time capsule firmware has also been bumped from 7.4.1 to 7.4.2. Screen Sharing from within iChat is another useful alternative to Back To My Mac in certain circumstances also. With my MobileMe account re-enabled I re-ran my matrix of tests. Turns out Apple have made quite some steps forward and most things actually "just work" now! Screen Sharing works all over. Finally.
The only remaining problems are file sharing from site A to the Time Capsule at site B and file sharing at site A being inaccessible to site B. I didn't test at sites C, D and E yet, until I see what Clarke comes back with. Let's hope he's Superman!