There is no way that a "federal Commonwealth" is ever going to happen.
The Commonwealth Realms (the 16 independent nations that have Her Majesty as Sovereign) are similar, but we are also very different. I doubt that any of the Realms would agree to relenquish parts of their identity, Sovereignty or independence to form this new country. This was the whole point in our countries becoming independent Realms.
While I do support, and hope for, closer ties between Commonwealth countries, especially the Realms, I cannot see this (federal Commonwealth) being any more than a silly dream.
Personally I lament the break up of the Empire, but lets face it the Empire is well and truely dead. We need to focus on maintaining and building on the links between our countries, but we need to be realistic about it.
Britain is becoming part of Europe, Australia and New Zealand are looking increasingly to Asia/Pacific and Canada is to the USA. We are no longer motivated by our shared heritage, but by regional factors.
Surely it is better for Monarchists to put our energy into something that is going to preserve the Monarchy, not undermine it. The FCS is open to a republic; that is reason enough not to support this proposal. Why would we want to trade in our current independent Monarchies for a federated republic? Why would we want to have a president of the Federal Commonwealth, instead of Queen of Australia, Queen of New Zealand, Queen of Canada etc.
Lets not forget that the Commonwealth has only remained because of The Crown (more specifically because of HM The Queen). If Britain were a republic, when the Empire began to break-up, we would not have had a Commonwealth...we may not have even had an Empire in the first place!
In short my point is that we should be looking at realistic ideas that will strengthen our links, as a brotherhood of nations, instead of indulging in fantasy...especially a fantasy that could threaten the very institution we all want to protect.