Not everyone claims that. God’s love and mercy are beyond our understanding, and to say they are infinite is a limitation, rather we may say they are truly ineffable.
Nevertheless, we know certain things - truths that the Magisterium has studied in centuries of work and that has kindly summarized for us in what we know as the Catechism. We can find in the Catechism the basic teachings that will allow us to grasp the mystery of salvation.
We know, then, that the Sacrament of Reconciliation was instituted by Christ for a reason. The Lord had already instructed us to ask the Father to forgive us our trespasses. If this prayer would remit our mortal sins, then Christ would not have given His Church an additional Sacrament, nor would he have given His apostles the authority to remit and not remit sins - each of us would have been able to obtain our own absolution.
Clearly, it is not so.
Even St. Paul, in the very early days, answers that if one is conscious of mortal sin, then one is not properly disposed to receive the Bread of Life, and if one does so anyways, then one is guilty of sinning against the body and blood of Christ - words of St. Paul.
If it is evident that the mystery of salvation is not fully revealed to us, how can we know what God would do if a person in a state of mortal sin would die? Is that not entirely up to the soul and the merciful Lord? If the soul has willfully rejected God and lost sanctifying grace, it is possible that our merciful Lord will be willing to forgive her, however the question is whether the soul would be willing to accept such forgiveness. If the soul had repented, we can presume she was touched by actual grace, and that fills us of a blessed hope that she will not reject the Lord but may have to undergo the purification of Purgatory.
Holy Communion is Christ that gives Himself. Nobody, even if one just confessed a second before receiving the Blessed Sacrament, is worthy of receiving the Eucharist. In this humble knowledge, we try to maintain the best possible disposition, and certainly receiving the Lord while in a state of mortal sin is far from being the best possible disposition. We will then abstain, pray the Lord to mercifully grant us a spiritual communion, and remember that “men live not on bread alone but on every word that comes from the mouth of God”.