As to Gregorian Chant, it had been falling out of favor for decades, if not several centuries before Vatican 2; OraLabora is a better source than I am. Having had experience in Gregorian Chant (although nowhere near as much as he) I can say it is by no means at all simple. Neither is 4 or more part harmony such as is required by Palestrina, who also wrote Masses centuries ago, and which music is in limited use today.
This is true, chant had become denatured by the 19th century, and hardly recognizable. The monks of Solesmes in France worked hard to get chant back on track by going back to ancient manuscripts and under the patronage to Pius X, Gregorian chant was standardized with the release of the Graduale Romanum, Vatican Edition, in 1908.
What we hear as Gregorian chant today is a 19th century interpretation of what we think chant sounded like in the 10th century. Nobody knows for sure and the ancient neumes have no staff and are very difficult to interpret.
In essence, Gregorian chant as we hear it today is about 120-150 years old. But it is standardized, although each choir has its own “voice” or sound. But they are all reading the same music. It’s also a dynamic field of study and in recent years, there has been much development, though all for the Divine Office both monastic and Roman. No changes for the Mass since the 1974 edition of the Graduale Romanum.
As for its performance you are correct, it isn’t easy. Some simple settings of the Ordinary are quite easy but the Propers, and the more complex settings of the ordinary, require some work. Our own schola is 20 years old now (I’ve been a part of it for 15 of those 20 years) and there’s still plenty of material we simply won’t touch, as it is too difficult.
As you point out, the documents of the Council are high-level. For the liturgy, Sacrosanctum Concilium laid out the general guidelines; the only immediate liturgical act in SC was the suppression of the office of Prime with immediate effect.
We know the the use of the vernacular took off like a wildfire. The context in which SC was written was one where Latin was still taught in schools. For the most part it no longer is. The bishops are working in 2018 realities, not those of 1962.
We also know that SC recommended that Gregorian chant have “pride of place”. But it did not say “in all places at all times” and SC allowed for other forms of sacred music. To have it take pride of place in any given parish would require that there be enough people interested in investing in the required skills; parishioners will soon tire of poorly done Gregorian chant, so it will take effort. Parishioners simply wishing for it won’t make it happen. Fortunately there are places where it has “pride of place”: monasteries, other orders/congregations, and scholas like ours who take an active interest (we’re not tied to one parish but rotate around the small city we’re based in).