Hey,
I haven’t posted alot on these forum (actually this is only my second post

), but I do feel I should make a contribution here.
The most important reason why a priest cannot alter the words of the Eucharistic Consacration (of the Latin Rite) is plain and simple that it would render the Consacration invalid. These are the exact Words handed over to us by Jesus Christ and trough the Magestrium of the Church we know them to be true. To alter them in any way, shape or form is to invite heresy, and to invoke damnation on all those parishioners who are deprived of his Blessed Blood and Body.
I know from personal experience that once some leniency is given to individual priests in the liturgy, they will soon alter everything they do not entirely endorse or (in most cases) what their parish members don’t like to hear. As such, in most Belgian parishes, we only have two Readings, no Psalms, no Gloria, no Eucharistic Prayer etc. Worst of all, sometimes the Holy Gospel is altered: “who wants to listen to an ancient book in grandfather-Dutch anyway? Better make it appealing to the younger generations” is their chief argument. Which doesn’t make any sense, really, because the only person whose age is below 50 is me and my brother (both aged 22) - and we’re both of us extremely frustrated. Worse still, there is no more room for the sacrament of Penance (God loves everyone, God is love, we should help poor people by giving them money, the Host is just a symbol - no need for Penance before partaking in Holy Communion, blabla) or of Eucharistic Adoration. In short, they are borderline heretical. And the bishops don’t intervene.
The result is a faith that doens’t appeal to people because it no longer puts God central (75% of Belgian priests do not believe in the transsubstantiaton) but themselfs and the material needs of the population. I live in a country that used to be the most Catholic part of continental Western Europe, and now has a church attendancy rate of about 6%. Out of which most are neo-lutheran liberal Catholics.
All of this started with the liturgical experiments in the sixties. And they too started with the alterations of a few simple words. Therefore, beware of even “slips of the tongue”.
My gloomy two cents

.
GDT