No, you are the one who is missing the point.
You are basing your whole case on your belief, which as far as I can tell is not grounded in anything except an assumption, that the only criteria for eligibility is full time employment. That may have been the case for the plan you turned down. That does NOT guarantee that it is the case for the plan currently in place. You do not KNOW that to be the case. You ASSUME that to be the case.
And unless we KNOW what the plan document says, and yes, there is a plan document whether you've seen it or not, then no one here, not me, not Beth, not anyone else, can tell you if your employer has violated any laws or not.
Now, either find out for CERTAIN what the plan document says and come back and tell us so that we can answer your question, or take your question elsewhere. I'm not wasting any more of my time trying to explain this to you.