Well, the text of the card is:
"Raven. Any effect that would make it Summer or Winter is considered inactive while The Raven's Song is revealed."
So it's not actually saying "effects that change the season don't work." It's saying "effects that make it one season or another are inactive."
By specifically making these effects inactive, it includes the constant effects that are already in place, not just the ones that have yet to be played. So Example #1 is the correct interpretation.
For those in the group that are probably getting caught up in the "would make it" wording, remember that constant effects (like "It is Summer" on the Black Raven) have no point of resolution. You play the card and it essentially creates a new, ongoing game state. Because this isn't an effect that is initiated and resolved (here's a good indicator of that; could you cancel it being Summer when the BR is played - assuming you had a Raven-esque cancel?), it is working all the time. As such, it "would make it" Summer every moment it is in play.
Anyway, the fact that they bothered to give the plot the Raven trait pretty well tells you it was meant to counter the "immune to non-Raven cards" ravens already in play. (Since a Black Raven's immunity is not active until it was played, the plot wouldn't need the Raven trait if all it was supposed to do was stop you from playing one of the attachments.)
If the plot was just supposed to stop a season from being changed, but keep the current season as Summer or Winter, it would probably use a simpler "seasons cannot change" format.