Not yet accepted - there is a contradiction here somewhere (or so it appears to me, the reader):

Yes, an existential forgery means that an attacker can produce a tag for an arbitrary message (an arbitrary "modification" includes a completely changed message, of course).

But here, you start the sentence with "but the only signatures the attacker can successfully forge are for ...".
The "the only ... the attacker can ..." implies that there *must be* other signatures that the attacker *cannot* (successfully) forge. But "are for modifications of messages that *we sent and signed*" is no restriction at all, as

* "modifications" include all arbitrary changes to modifications; 
* "that *we sent and signed*" is irrelevant as a restriction, as this is not known to the recipient other than by signing;

So I *thought* that the restriction was for extensions. If it is not, what is the "but the only" for?