開封 and 開ける: unsealing and opening in a broader sense
Compare opening a sealed box or envelope with opening a closed window. Focus on the seal without adding a requirement that the object has never been opened before.
Open a sealed object
開封, read かいふう, means opening or unsealing. 届いた箱を開封する前に宛名を確認した。 says that someone checked the addressee before opening the delivered box.
The completed action is checking the addressee. Preserve the order given by 開封する前に rather than reporting that the box has already been opened.
Open a closed window or another object
開ける, read あける, means open. 暑いので窓を開けてもいいですか。 asks whether the speaker may open the window because it is hot.
This asks permission to open the window. 開ける covers opening more broadly than breaking a seal, and this question does not report an already open window.
The seal and the requested actor
この封筒は本人が開封してください。 asks the person concerned to open the envelope personally. 本人が specifies the person requested to act.
開封 focuses on opening a sealed object. The word alone does not establish that it has never been opened before, and the request does not establish that opening has been completed.
Does 開封 necessarily mean that the object is being opened for the first time in its entire history?
Show the answer and why
No. It focuses on opening the sealed object. Any previous opening history needs separate information.