There is no contradiction between Divine Law and Church Law. The Divine Law commands we offer sacrifice to God. In the New Testament, such sacrifice is Holy Mass.
But Pax, no one ever said there is a contradiction between Divine and Church Law. Divine Law commands we keep the Sabbath Day holy. This law can never be changed. The Church commanding that we go to Mass on that same day is in complete harmony with Divine Law.
The Church could change that law she made so that we have to attend Mass twice on Sunday, or both Saturday and Sunday or whatever. The Church's law makes it a sacrilege to receive communion more than once a day - yet she allows it twice in the same day for certain cases such as for Viaticuм. Her laws may be changed, Divine Laws can never be changed.
Anyway, that's the way I discern the difference between the two.
It is a mortal sin to miss Mass, whether Church or Divine Law makes no difference.
The old law required an annual sacrifice for sin; similar to our annual requirement to go to confession once a year. But could the Church get rid of this annual confession rule? I would say ‘no’, because it has its roots in the Old Testament, which came from God (ie Divine Law).
The Church could change the rule to make confession twice or more a year, or once every 13 or 20 months or whatever. This would mean the Church's old law would have to be abrogated, so in that sense she could get rid of the annual confession rule.
In the same way, the weekly requirement for Sunday Mass is a Church Law, yes, but does that mean it ORIGINATED with St Peter? In other words, is the Sunday Mass requirement a creation of the Church, which could be done away with entirely? No, it cannot. The requirement for Sunday Mass would have to be considered part of Tradition, ie an order from Christ to the Apostles, so it is Divine Law, made know through the Church.
.
Holy Days of obligation are strictly Church laws and almost all (except Christmas, which feast has been celebrated since Apostolic Times) could be, in theory, removed from the calendar.
I do not know when Our Sunday Obligation originated, but other than to satisfy idle curiosity that is irrelevant because it is a law binding on all Catholics under pain of mortal sin, this is true in most cases, but there are exceptions.
We all know that there are many trads who have no access to Mass at all, or perhaps have access only once every third month on a Wednesday - in this instance they do not sin by missing Sunday Mass and are not bound to that law because it is the Church's Law. Although for their own good, they had better do whatever they possibly can to not miss that rare Mass on Wednesday when it happens.
Because it is the Church's law, it is up to the Church to provide the Mass for us to go to, or it is no sin. In such cases when there is no Mass on Sunday, it is therefore impossible to go to Mass on Sunday and we are not bound to Church Law, but we are still bound to the Divine Law and must do what we can do keep the day holy on our own.
Another exception for example is if the chapel makes your presence conditional, i.e. that you can participate in the Mass but only under certain conditions, such as you must first sign a paper, or if you must first give a verbal affirmation to the priest or chapel rep that you deny EENS, or admit there is no pope, or reject certain doctrines, or whatever - then you must not attend and in those instances are not bound by the law.
It should be noted for poche and other unknowing lurkers that all of this pertains strictly to the True Holy Sacrifice of the Mass, and most certainly does not apply to the NO's Sunday service.