I agree. Each public key creates a different encoding, or a different language, as it were. These encodings/languages are all related, but mutually incomprehensible. Encryption software has the capability to read any of these languages because it is multi-purpose software.
One possible hole here is that since they share a commen algorith then the algorithm is the 'language' and not the actual messages. This would mean that you are each using the same language. There is also the aspect of once discovered you could be charged with obstructing justice which has very stiff penalties.
Here, then, is the connection back to the original issue. The courts distinguish between acts of speech (fifth amendment protection) and supplying objects, such as a subpoena provide the key to a safety deposit box. As Marc Rotenberg once put it to me, the court cannot require you to incriminate yourself, but they can require you to participate in your own downfall. Forward secrecy protects you against court order, because you cannot be held in contempt of court for not providing something that doesn't exist. If you destroy your keys in a timely fashion, your exposure is limited to the time since the last key change.
They make you participate by giving you immunity in which case you have no choice but to reveal it or go to jail. Either way somebody is going to jail. As to self-incrimination, gee, I thought that was the whole purpose of calling witnesses and such, either to discredit themselves (which is equivalent to incriminating oneself if you are the defendant) or to incriminate others (and here we are back to immunity). While it is true you can't be held in contempt of court for not providing something that doesn't exist they can get you for destroying evidence.