Pope Pius XII sees much to affirm in the 20th century’s effort to open understanding of Catholic liturgy to the laity. What strikes me in MD 5 is the use of two terms linked to Vatican II: fountain and participation.
5. The majestic ceremonies of the sacrifice of the altar became better known, understood and appreciated. With more widespread and more frequent reception of the sacraments, with the beauty of the liturgical prayers more fully savored, the worship of the Eucharist came to be regarded for what it really is: the fountain-head of genuine Christian devotion. Bolder relief was given likewise to the fact that all the faithful make up a single and very compact body with Christ for its Head, and that the Christian community is in duty bound to participate in the liturgical rites according to their station.
I’d say that the Mass is more than an inspiration for “Christian devotion,” though it certainly is that. The post-conciliar view would be that the Mass is the “fountain-head” for the whole Christian life (Cf. SC 10).
And we have a rather strong reference to participation. Note that such activity is not described as being for individuals, but is an action of the entire faith community. Lay people do what is expected of their role, but the base expectation is that they “participate in liturgical rites,” they are not spectators and passive recipients of grace.
Any serious examination of participation in Mediator Dei is going to have to work from that baseline.