My thoughts have always been if you do not believe in transubstantiation (which is the biggest of the biggies), that would automatically make you not Catholic. Catholics believe in the true presence. If one doesn’t believe in the true presence, how can one call themselves Catholic? Catholicism is not a cafeteria, where one can pick and choose their own beliefs, rules, etc.
Also, believing in and practicing things such as birth control, which is strongly against Catholic teaching would qualify a person for excommunication.
My understanding on excommunication is that it’s not something that is bestowed upon you in any formal manner by a priest, bishop or even the Pope. Excommunication is an automatic consequence of not following the rules of the faith. In other words, we excommunicate ourselves by practicing non-Catholic behavior. Coming back into full communion with the Church would simply entail a change of heart on the matter or matters, and the sacrament of reconciliation.
If one is excommunicated, that person would still be welcome at Mass, etc., but if using birth control or not believing in the true pesence of Christ in the Eucharist, would not be welcome to receive Communion. Then again, why would that person want to receive the Eucharist if they didn’t believe in the true presence. When you go up to communion, you are, in fact, saying “Amen” (so be it, or I believe) to the words, “The Body of Christ”, not the words “The spiritual presence only of Christ”. That person’s acceptance of Holy Communion would be a lie and hypocritical, to say the least.
I’m speaking from the heart, so if anything I said is theologically incorrect, I apologize. This is how I learned it growing up Catholic. Hope I was helpful.