The “Orthodox Church” is not recongized as a Church. Rather their particular Churches (bishop and flock celebrating the Eucharist) are recoginzed as particular Churches. While there are many particular Churches (some separated), there is only one Church of Christ, which is permanently tand concretely the Catholic Church.
From what I understand, the Church teaches that with regards to members of the Orthodox Churches that may be saved due to being in good faith and not guilty of schism (provided of course they are not guilty of other mortal sins of course), they are indeed sanctified by those elements of sanctification in the Orthodox Church (such as valid sacraments). However, this is by virtue of the person’s connection to the Catholic Church created by baptism and that those elements of sanctification originate with the Catholic Church, properly belong to the Catholic Church, and draw their efficacy from the Catholic Church. Therefore, it isby their relationship to the Catholic Church that such individuals are saved, not directly be their membership in an Orthodox Church. It’s a subtle distinction, but an important one.
What was the name of Christianity before the Great Schism? Was it already called the Catholic Church? Where did the term “Orthodox” come from?
The Church was called Catholic and the faith was called orthodox.
If the original Church split in half, how do you know which Church is the original one?
For this very reason, Our Lord established one See as an immoveable rock–that is, the See of Peter in Rome. Where that See is, there is the Church.
Also, are various sects with apostolic succession, such as SSPX or Sedevacantist groups considered churches or ecclesial communities? And is there salvation in them? Are such groups on par with the Orthodox in the eyes of the Catholic Church?
Most strains of sedevacantism are also a schism (since they reject communion with the whole Church) as well as submission to the Pope), but their members are less likely to be in good faith, since they have not been born generations later into the split, but usually made the decision themselves to split. Their bishops tend to be just “sacrament machines” rather than pastors, so they may not be considered particular Churches. The SSPX are members of the Catholic Church, but without legitimate ministry and under certain ecclesiastical penalties.
And out of curiosity, is the conversion from Catholicism to Orthodoxy as simple as the Catholic process in which one only has to recite the Creed to convert?
The Orthodox Churches are not united on this issue. They range from requiring rebaptism and abjuration of heresies to what you describe.