Take this for what it’s worth as I’m not a theologian.
But let’s take a Lutheran baptized as a child. The Baptism is valid, so by that very act the child, though Lutheran in name, becomes a member of the Catholic Church. If he dies the next day, he very likely will enter heaven.
But suppose he grows up. He is truly a member of the Catholic Church, yes, but he also does not have access to the full complement of sacraments. If he commits mortal sin, he, like all Catholics, will go to hell—but unlike Catholics he does not have recourse to valid sacramental Confession. So his contrition must be perfect—or he will be damned.
Also, is he guilty of mortal sin of heresy? Tricky. He may well hold to the Lutheran concords...but is it out of honest ignorance? Or is it out of malice? Does he know that what he believes is heretical? If not, there is no guilt. But if so, he can be damned by that alone.
I think this is what is meant by imperfect communion. The person is in fact grafted into the Church, but only imperfectly, in that he does not have at his disposal all the tools the Church has to offer.
And for that matter, by a mortal sin (Lutheran's do have them, but they don't quite mean what Catholics mean).
For instance, a mortal sin is where a person makes a conscious decision to turn from God. Not just having a bad desire or temptation, or even giving in to one in a moment of weakness.
In your description, most of what is being said in ecumenical talks would be at best invalid.
Why is it so hard for people to grasp this?