If a sin can be dispensed, is it really a sin? Question

During the Coronavirus pandemic many Catholic bishops have decided to dispense the faithful from the obligation of attending Sunday Mass.  The standard teaching is that missing Sunday Mass constitutes a mortal sin (according to things like CCC #2181).

Consider two kinds of mortal sin: murder and missing Mass.  The latter can be dispensed whereas the former cannot.  Common sense would seem to say that if something is a sin then it is a sin regardless of what the Church says.  Murder is a sin and the Church cannot "dispense" someone from the obligation to not murder.  But if the Church cannot dispense someone from a sin, then how can the Church dispense someone from the obligation to attend Mass given the catechism's admonition that to miss is a "grave sin."  If a sin can be dispensed, is it really a sin?  Or more precisely: If an act which is usually considered a mortal sin can become non-sinful with a dispensation, is it really a mortal sin?



What I am asking


If a sin can be dispensed, is it really a sin?
Does the Church have the power to declare that something is now a sin which was not previously a sin, and to declare that something is no longer a sin which was previously a sin?
Why are the sins of murder and missing Mass treated so differently?  What is the fundamental difference between the two?  Do they have the same degree of severity?




What I am not asking


I am not asking about what constitutes a mortal sin (e.g. knowledge, freedom, consent, etc.).
I am not asking about whether the Church has the power to judge whether something is or is not a sin.  For example, the Church judges masturbation and contraception to be sinful.  This question is not about whether She can judge something to be sinful, but whether She can dispense the faithful from an obligation to avoid sinful activity.
"If a sin can be dispensed, is it really a sin?"

No, a sin which is dispensed of could never be a sin, because a sin is an offense to God, and offending God can never be excused.

"Does the Church have the power to declare that something is now a sin which was not previously a sin, and to declare that something is no longer a sin which was previously a sin?"

No, the Church cannot dispense someone from their moral duty as creatures of God to avoid sinful activity.

However, the Church can remove the obligation to do something in a certain way (i.e. attend Mass as a species of activity which falls under the genus 'keeping holy or separate the Sabbath day'). By removing the obligation, they remove the possibility of contravening any law, because it is no longer binding.

In this case, the Church has not modified what is intrinsically sinful (excusing people from offending God), but have modified what Christians are bound by obedience to the Church to do.

To use @Geremia's example: if a parent tells their child not to eat a cookie, it is wrong to for that child to eat the cookie. If a parent then permits the child to eat a cookie, then it is not wrong for the child to eat the cookie. In this instance, eating cookies, or not eating cookies is utterly irrelevant - only the obedience to one's parents.

Similarly, every Christian has the moral duty to obey the commandments of the Church as their legitimate authority and teacher - and per the perennial trope, mother. Attending or not attending Mass is not intrinsically or inseparably related to keeping holy the Sabbath day, as is proven by the sheer fact that the Mass did not exist until Calvary - one thousand years after the commandment was issued by God.

Attending Mass is one of many ways to keep holy the Sabbath day, and thus is not necessarily commanded by the commandment to keep holy the Sabbath day (i.e. where Christians have always viewed Sunday as the new Sabbath of Christians).

Thus, the Church, in her prudence, or even simply by her authority, has the right to bind or loose the obligation to attend Mass on Sunday, without touching the sinfulness of obedience, which remains, whether one is bound by obedience to attend Mass on Sunday, or freed from the obligation by the Church. To date, the Church has not seen any good reason to dispense anyone from attending Mass every Sunday except for relatively extreme travel requirements, illness, or, apparently, in times of the current crisis.

"Why are the sins of murder and missing Mass treated so differently? What is the fundamental difference between the two? Do they have the same degree of severity?"

A mortal sin is simply any sin which leads to spiritual death (hence "mortal," which means "causing death"). This doesn't imply that all sins which are mortal have the same severity, incur the same punishment in hell (or temporal punishment, if forgiven). A child abuser will incur more punishment for his sin, than someone who elects not to travel to attend Mass because he has a bad case of the sniffles. For reasons that should be obvious. Murder is obviously worse than wilfully lying about a non-trivial matter (such as making a false oath). Homosexual fornication is obviously worse than heterosexual fornication, because it involves at least one more degree of deviation from God-ordained sexual relations than does heterosexual. Etc.

That is, while venial and mortal sins designate two manners of sinning, they don't imply that all venial, or all mortal, sins are the same as every other, or carry the same penalty.