Whether we are praying to the same God is assessed on a case-by-case basis. A special understanding is given to the Jewish people. Of them, the Catechism states, The Jewish faith, unlike other non-Christian religions, is already a response to God's revelation in the Old Covenant. To the Jews belong the sonship, the glory, the covenants, the giving of the law, the worship, and the promises (No.839). They direct their prayers to God the Father, as we do though they do not understand God as Trinity, or accept Jesus or the Holy Spirit as God. And yet, it is fair to say they do direct their prayers to the same God, though imperfectly understood, to whom we direct our prayers though more perfectly understood.
 
As to the Muslims, they are monotheists, and, as the Catechism notes, “These profess to hold the faith of Abraham, and together with us they adore the one, merciful God, mankind's judge on the last day.” (No. 841.)
 
The Church’s relationship with other non-Christian religions is less certain and variable. Of these the Catechism says that God is clearly the common origin and end of the whole human race (cf No. 842). Further, The Church recognizes in other religions that search, among shadows and images, for the God who is unknown yet near since He gives life and breath and all things and wants all men to be saved. (No. 843)
 
As to what the majority of people can expect, especially those who do not know or obey Christ, the Church expresses hope, but also warns that though their salvation apart from Christ is possible, that does not mean it is probable. For the Catechism also reminds that sin requires the remedy of faith, “But very often, deceived by the evil one, men have become vain in the reasoning, and have exchanged the truth of God for lie, and served the creature rather than the creator.” Thus, unbelief or imperfect belief is a form of darkness that hinders salvation and needs the healing of true faith.
 
So, in the end, there is only one God and all yearning for God is somehow directed to him. But there are often errors in the ways people of the world understand him. And thus it is essential for the Church to correct errors, and draw everyone to the one, holy, Catholic and apostolic faith in which there is the best hope for salvation.

No, grave and mortal mean the same thing in Catholic moral teaching. Hence, missing Mass without a serious reason is a mortal sin (cf Catechism No. 2181).
 
It is true today many moral textbooks and Church documents use the word “grave” more often than “mortal.” There are likely pastoral reasons behind this. For it was a growing tendency for many poorly catechized people to think “mortal” sin referred only to killing someone. So, there came the tendency to use the word “grave,” meaning “weighty or very serious sin” to refer to mortal sin. But, as you point out, this has led to other pastoral problems, wherein people do not often understand that grave and mortal mean the same thing.
 
Pope John Paul II found it necessary in 1984 to clarify that grave and mortal mean the same thing. He wrote: In the Church's doctrine and pastoral action, grave sin is in practice identified with mortal sin….There is no middle way between life and death. (Reconciliation and Penance # 17)