My counsellor / life coach taught me to make a distinction between "feelings," "perceptions," and "beliefs." She talks about the "stories" we tell ourselves to explain other people's behaviour.
Feelings are like happy, sad, mad, scared, etc. In contrast, beliefs and perceptions are more like the causes of these feelings. They're the things most people mean when they say "I feel lied to" or "I feel used." "Lied to" and "used" aren't "feelings." They're a perception of an event, an interpretation of behaviour.
The reason I'm going on about this is that it helps to separate the emotions you're feeling from the perceptions you believe caused those emotions.
You're feeling sad over the loss of something you had. That's perfectly reasonable. Now you're telling yourself a story that they deceived you, that they they portrayed their relationship as being stronger than it actually was. That's your perception of their behaviour.
Have you considered the very likely possibility that they, too, believed their relationship to be more stable than it actually turned out to be?
To me, deception is deliberate. I don't see anything in your description that leads me to believe they intentionally hid problems in their relationship just to lure you and your wife in.
It seems more likely to me that their relationship indeed was healthy and stable in the beginning, and because of that, they took each other for granted. Instead of making sure they spent time maintaining their connection, they assumed their marriage would just keep ticking. Many people fall into that trap. It's not just a poly issue. New jobs, having kids, making friends, going to school... all of these things take time and energy. If people don't make a point of fuelling old flames,
If I'm understanding your #4 correctly, you're saying that while he's attempting to reconnect with his wife, he doesn't believe he can be there for your wife. That seems reasonable to me. Part of their problem is that they've been spending their time and energy on you and your wife. In order to get their marriage back to what they'd like it to be, they need to focus on one another and spend their time and energy on each other. It's equally reasonable that this would make your wife sad. It's a loss. Loss requires mourning.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker