Understanding Openness
About that open business:We start with the adjective lexeme OPEN, which is a pure stative; The window is open doesn't require that it was ever closed (it might have been built that way), and The restaurant is open doesn't require that it was ever closed (it could be one of those restaurants that are always open). The adjective can serve as the base for deriving two verb lexemes, the inchoative OPEN 'become open' and the causative OPEN 'cause to become open'. The story of the PSP opened then goes much as for the PSP closed, but with an important difference. The PSP opened has a passive use, as in The gate was opened by the guard at dawn. But the stative adjective use is hard to get: The gate is opened at the moment is decidedly odd. Why?
Because English already has a way to express this meaning (and a way that's shorter and less complex than the PSP opened): the adjective open. The PSP opened in this use is PRE-EMPTED (or, if you will, PREEMPTED) by the simple adjective open. (Pre-emption is a perennial topic in morphology and lexical semantics. A textbook example: English has no causative DIE alongside inchoative DIE because it's pre-empted by causative KILL; in a sense, KILL got there first, so there's no point in creating causative DIE.)
But... in special circumstances, the PSP opened could be used as an adjective -- with the semantics of the passive, as for disputed above. In particular, The envelope is opened could be used if the envelope was not merely open (rather than closed or sealed), but gave evidences of having been opened, say by slitting with a letter opener. This is a case where open might not be specific enough, so it doesn't automatically pre-empt opened.
Got that?