Firstly, I think you simplify sinlessness. It is so easy to sin, it only takes a single thought to
deserve Hell. God’s grace is manifold, but, I think it is nessasary to stress human sinfullness.
I think you underestimate the Mercy of God, or overestimate how easy it is to fall into mortal sin (which is the only kind of sin that can cause you to go to hell - a trillion venial sins won’t do so).
I don’t think you’ve thought that one through. An excommunicate is guilty of at least one mortal sin: serious open and willful disobedience of the Church. If a person wishes to gain absolution from mortal sin, s/he must make a good Confession, which would include repenting of the disobedience to the Church. But if the person does that, the excommunication can be lifted! Problem solved.
People who are in open and willful rebellion against the Jesus’ Church are in mortal sin whether the Church actually excommunicates them or not (and, these days, She rarely does). But, either way, this sin must be repented for the person to avoid hellfire.