Can't imagine how priests would even manage or everyone didn't fall asleep.

Well, one of the most popular collections of sample sermons from the 14th century was called "Dormi secure"...Sleep Securely.

...But actually, it's the priest who can "sleep securely," the title continues, because he knows he has a good sermon to preach the next day.

The medieval Church was, in fact, concerned about doing a good job with Mass. And yet, all the points you listed were either a little or a lot true. The apparent contradiction comes from the fact that the purpose of Mass changed. Both from the 11th century to the 15th, and from the 15th to today. A lot of what looks like a problem from today's point of view, wasn't.

And as for what was a problem--and for priests who didn't care or who were genuinely bad at their jobs--the Church came up with theological solutions to get around it.

First let's set the scene for a medieval Mass in general. Everyone crowds into the church--there are no pews! The priest stands at the altar, facing away from the audience. He performs the whole service in Latin. The lay people are most likely not receiving the Eucharist, not even the bread. If there is a sermon, it takes place after Mass (or possibly not connected to Mass), generally outside the church itself, and is likely in the vernacular.

Now, the 1000s.

Things are pretty disorganized. There's nothing really resembling a parish system. Cities have cathedrals; lords have (or build) churches; queens have their own chaplains (appearing very devout was quite important for queens' image at this time). But this doesn't leave peasants stranded. What we call the "proprietary church" system is basically lords building semi-local castles for peasants and hiring priests. Yes, hiring.

So you have a priest, who might just be the nearby person willing to do it for the lowest salary, saying Mass maybe once a week. Do people come? Yes, even though the priest is speaking Latin, even he might not understand what he's saying, they don't receive the Eucharist, and there's no sermon. The idea was: (1) the priest consecrates the Eucharist, and then receives it for the people/in place of them. (2) Attending the Mass was a spiritual benefit, almost like a charm or a spell of protection.

If the idea of the priest taking the Eucharist on behalf of lay people seems odd, consider: at infant baptisms, adults make the vows on behalf of the children!

This is Christianity, though, make no mistake. People stand as parents and godparents for those babies' baptisms, at which they have to recite the Our Father and probably the Nicene Creed in place of the child. A good marker of the Christian-ness of this era is at the end of the 11th century, when peasants leave everything behind to tag along with the First Crusade. (The crusader army is not happy about this, and tries to ditch them several times.)

The 1200s

In the years around 1200, we see: (1) a new devotional focus on the humanity of Christ, which among other things plays out in devotion to the Eucharist (2) independent religious movements that the Church has to either absorb or fight as heretical, which means the Church is in search of more control (3) people at all levels of society are much more religious than before.

So at the Fourth Lateran Council in 1215, the Church passes a major, major, game-changing rule: every Christian of both sexes must confess once a year (not at Mass), in preparation for taking the Eucharist (bread only) once a year on Easter, at Mass. This rule is probably what your source was thinking of as "attendance is mandatory."

It's going to take a long time for that rule to be universally followed, of course, but 1215 is a good marker date. Some other game changers are happening around 1200, too.

Books are still really, really expensive, and Bibles are long enough that they are even more crazily expensive. However, cathedrals and an increasing number of parishes have texts called plenaries instead, which lay out the Gospel and epistle readings for the year's Masses in order. In other words, what is useful!

First, the new universities are working hard to churn out priests. Most university students aren't debating intricate theological questions. They're showing up to learn basic grammar, rhetoric, logic, and the Bible; the goal is a career in the Church or as a "clerk" in the growing government bureaucracies. So there are still loads of uneducated priests out there--and will continue to be--but the stock of good ones is growing. And weekly Masses continue to be treated like a "good meal" or a good charm for the week.

The stock of good priests who are also preachers is growing, too. The new Franciscan and Dominican orders of friars are founded on, among other things, a mission of preaching to the public. They are also priests who can and do perform Mass in their own churches. People still have to confess and take the Eucharist from their "parish" priest (part of the Lateran IV declaration). But they can especially hear sermons from the friars--sermons in their own language.

Sermons aren't just about the sermon, though. Think of the big crowd (although probably outside, as mentioned above). This is the time that people have to chat, gossip, hear the news, fight.

The 1400s

By now, the Lateran IV rules of confession and the Eucharist are basically in place everywhere. Confession isn't quite "public" in the sense you might be thinking of, especially given the whole crowds in churches scene. However, the idea of a confessional is still 100 years away. People stand in a line waiting for their turn, and straining to overhear what the people in front of them are saying.

Mass looks a lot the same--Latin, priest turned away from the crowd, no pews. But while the priest is still receiving the Eucharist on behalf of the people, some dioceses are permitting or requiring churches to offer it to the people two, three, four times a year.

Sermons are a big deal, bigger than ever. Why? Because the 15th century is really the first time in Christian history that Christianity becomes a religion in our sense: a set of beliefs that people learn and assent to. Priests are also encouraged to, in their sermons, interpret the day's (Latin) Bible reading for people in the vernacular, or even just straight-up read it.

The availability of paper, and eventually the printing press, make books a LOT cheaper and more accessible. Adult literacy is also rising, although by "rising" consider that we still mean maybe 15% overall. Parishes have plenaries, maybe even a Bible...in Latin OR in the vernacular. In cities, wealthier people are also starting to own plenaries and Bibles, and even take them with them to Mass and sermons.

I stress, of course, that these are all very broad and stereotypical pictures; on the ground, situations change from church to church and year to year and priest to priest and individual lay person to individual lay person. Complaints about bad priests (anticlericalism) is essentially an industry in the late Middle Ages.

But it's a general overview. And I hope it answers your underlying question of how the Church can seem so unconcerned with failing people. It was unconcerned precisely because, on average, it wasn't failing them.