How long has Picard been the focus of the Changelings? Picard's first call would be to Riker. That's no real secret. And since the Changelings had access to Shaw's psych eval, couldn't Riker's also be available? His son's death hit him and changed him, possibly making him more vulnerable to a family threat.
I think it's possible that the real Deanna is prisoner and a possible changeling dupe is wherever. It's also possible that the "prisoner Deanna" is a changeling to fool Ryker. Personally, I think a real Deanna would complete the NCC-1701-D color group and continue the love letter to fans. I can't believe Deanna is only showing up for series finale hugs. Keep in mind that the season is 6 down with only four to go and the fan service seems to indicate a lot of resolution and potential futures for characters in the finale.
My next prediction: I'm expecting Picard needing to fire at the shrike with Ryker aboard in a reversal of Best of Both Worlds.