From what I understand from Paul’s writings, during baptism we receive the Holy Spirit and are thus ‘in Christ’.
I have heard that the Holy Spirit never leaves us, even if we’re in mortal sin, because of baptism…therefore to me that seems to suggest that we cannot lose our ‘in Christ’ status, because the Holy Spirit remains in us even when we’re in mortal sin.
Can one lose their ‘in Christ’ state whilst in mortal sin?
If so, then how is being ‘in Christ’ conditional, whilst the indwelling of the Holy Spirit is unconditional although they both relate to one another?
But then also how can one be ‘in Christ’ and be in sin at the same time?
The Holy Spirit ( indeed the Holy Trinity) is no longer indwelling when we fall into mortal sin. Of course the Holy Spirit and the Father and the Son are present everywhere …but we are discussing the special indwelling when a person is in a state of Grace.
So no the Holy Spirit does not “remain in us” in the sense you mentioned. Only in the sense that God is indeed everywhere.
As to “in Christ” -one can say that one who is in mortal sin is no longer “living” “in Christ” - they are a member of the Body of Christ -they remain in a sense “in Christ” - in that they are baptized but they are not at the moment “living”.
The Pauline use of the phrase I think usually would more directly refer to one who is “living”. Though he would likely have noted that yes one who has committed mortal sin is still in Christ in the other sense. It is just that when he usually uses that term he is referring to Christians who are “living” in Grace.
Thankfully God desires to give us life more than we desire to commit mortal sin. Let us repent and seek to return to God if we should fall into such.
There are two words that are helpful to remember when talking about baptism, which are character and grace. The character of baptism is like a permanent stamp on our soul that we receive when we are baptized. This mark always remains, so we only are ever baptized once. On the other hand, the grace of baptism is what sanctifies us and makes us right before God is dependent on the disposition of the recipient. Someone who receives baptism insincerely receives no grace at all. Someone who receieves the grace of baptism and sins mortally afterwards loses the grace of baptism.
It applies to all who are baptized in the faith for he is referring to the infusing of the Holy Spirit into the soul of the baptized. Since Christ, the Father and the Holy Spirit are one, being baptized in the name of the Trinity puts an indelible mark of grace on the one baptized.
In cases like this, in which it is not clear from a “cold” reading to whom Jesus is referring, a good Bible commentary is invaluable.
If Jesus meant that only his immediate disciples, to whom he spoke at that particular time, could have him dwelling within them, it would make no sense for him to have commissioned them to:
Matt.28 Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit,  teaching them to observe all that I have commanded you; and lo, I am with you always, to the close of the age."
To understand Catholic teaching about these sorts of things we need to see them within the context of the whole of Scripture and Sacred Tradition. The Church has done this. Indeed, it is her duty to do so, which is why we can know that what she teaches is correct concerning the teachings of Christ–apart from his promise that the Holy Spirit would guide his Church into all truth.