Yes, the Catechism of the Catholic Church says, The faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin (# 2181)
 
The Lord Jesus also warns, If you do not eat the flesh of the Son of Man and drink his Blood, you have no life in you (Jn 6:53) Thus, to miss Mass and stay away from Holy Communion is a form of spiritual starvation. Further, we fail to give God the praise, worship and thanksgiving He is due.
 
It is a sad fact that this precept and moral teaching is underemphasized today. Priests, catechists and parents must be clearer in teaching and witnessing to this requirement rooted in the Third Commandment. They must also teach why.
 
There is a modern tendency, not wrong itself, to emphasize “positive” reasons to do things rather than simply quoting laws. But the gravity of the offense against God’s Law should not go unstated. Further, obedience to God’s law is of itself good, and brings with it many benefits and blessings such as the instruction in God’s Word at Mass and the astonishing blessing of being fed on the Lord’s Body and Blood.

There are important differences between the practices. Relics usually involve small portions of the body, such as bone fragments to be reserved for veneration. Thus the entire body of a saint is not “scattered” throughout the world, or even scattered locally as with strewn ashes.
 
Secondly, the relic of the saint is retained for veneration as a kind of physical and visual memory, whereas scattered ashes are spread in order to disappear and return to the elements. And while some may find this meaningful, the result is that any physical reminder of the person is lost, quite different from a relic.
 
Thirdly, with a relic, the physical presence of a small portion of the body is treated with reverence, much as a gravesite would be, and prayers are often said in its presence in acknowledgment of the given saint. In the case of scattered ashes, neither the ashes nor the place of their dispersal receive the same kind of veneration, and may in fact be tread upon by human beings unaware of their presence, and by wild animals.
 
While it will be admitted that burial practices have some variance across cultures, the current practice of the Church, out of respect for ancient Christian practice and current sensibilities, is to insist that human remains of any sort be buried or entombed. The Order of Christian Funerals has this to say about the disposition of cremated bodies: The cremated remains should be buried in a grave or entombed in a mausoleum or columbarium [a cemetery vault designed for urns containing ashes of the dead]. The practice of scattering cremated remains on the sea, from the air, or on the ground, or keeping cremated remains in the home of a relative or friend of the deceased are not the reverent disposition that the Church requires.

Regarding baptism, the catechism affirms the following: Baptism seals the Christian with the indelible spiritual mark (character) of his belonging to Christ. No sin can erase this mark, even if sin prevents Baptism from bearing the fruits of salvation. Given once for all, Baptism cannot be repeated. (#1272).
 
That said, your question also seems to touch on the question of how deep one’s communion is with Christ and his Church. And if one were to reject the teachings you mention, they would, in effect, seriously harm, even sever their communion, their unity with the Church. At times the Church must ask those who intentionally dissent to assess their own communion with the Church, and to no longer celebrate a communion that is seriously impaired by refraining from receiving Holy Communion. In rare cases the Church may see a need to formally declare an excommunication exists.
 
However, even in such cases, given the indelible mark that baptism confers, one can never utterly lose the status of belonging to Christ. By analogy, even if a son or daughter of yours were to wander far from you, live in total contradiction to what you believe, and even curse you to your face and act so badly that you had to erect legal protections for yourself, none of this would change the fact that they are still your son or daughter.

When an artificial device such as a knee replacement is used, or in the case of an organ transplant, we are seeking to repair something which is no longer working properly. However, in the case of contraception we are seeking to render dysfunctional, something which is functioning properly, and is a normal aspect of a healthy body. This is a rather big difference and renders your example more of a contrast than a comparison.
 
Further, it is too simplistic to say that the Church condemns artificial birth control merely because it violates God’s will in our lifecycle. It is more proper to say that the Church condemns artificial contraception because it violates our obligation of safeguarding both the unitive and the procreative dimensions of the conjugal act. In other words, contraception violates the intrinsic meaning of human sexuality.
 
The replacement of a knee or a kidney however, does not violate the essential meaning and purpose of the body. Rather, it helps to enhance the body’s overall function which has been diminished somehow, either by injury or disease.
 
There are of course limits to bodily interventions that we might make. There should be good reasons to replace organs or body parts, and our interventions should enhance the proper, God-given functioning of the body, not alter its intrinsic meaning. There are increasingly strange practices today involving exotic piercings, and extreme “body art,” some of which come close to mutilation and which may hinder the proper functioning of the body. So called “sex change operations” would also be excluded since they seek to fundamentally alter what God has given.
 
However, other things being equal, it is not inappropriate to make proper medical interventions to ensure proper and healthy functioning of the body.