On the changes to the Mandatum: these are all questions which are out of our league ("well above our pay-grade," as Father Hunwicke would have it).
I, personally, suspect that this was originally a clergy-only function, performed by an ecclesiastical superior, which would preserve both the abasement and institution of the priesthood motifs (see comments to above link), both completely out of step with our "moderne" age.
Ignoring Petrine prerogatives, what could have motivated the following, however: the assignment of the prime significance of "the boundless love and Mercy of God to all, and not least to those on the peripheries of Society"?
All Western rites preserve the Epistle reading from I Corinthians 11. But they differ on the Gospel as well as the other lections.
- The Ambrosian rite commences with a synaxis, featuring:
... a lection from the Book of Wisdom (2:12-3:8) that speaks of the plots of the wicked against the "just one," who troubles their numbed consciences by His teachings ... Another of the readings, from the Book of Daniel (13:1-64), tells of the leveling of false charges in court against the Jewish woman Susanna by two lecherous judges, whose sinful advances she had spurned; Susanna's innocence is subsequently revealed to all present through the intervention of the young prophet Daniel. The application is fairly obvious: For just as Susanna was put on trial and falsely accused for remaining faithful to God's commandments, so will Christ be bound, taken to court and falsely accused for teaching His Father's commandments ... There is also a Gospel reading at this morning service: Saint Matthew's poignantly terse account of Judas' meeting with the chief priests (26:14-16), during which he asks the terrible question, "What are you willing to give me if I hand him over to you?"
- The Gallican rite uses a related Gospel pericope: Matthew 26:20-35.
- The Mozarabic rite has as the first reading Zechariah 2:13-3:5; 11:7-14; 13:7-9, which is very, very interesting (refiner's fire), as well as emphasizing the obvious intertextuality.
The Novus Ordo RCL adds Exodus 12:1-8, 11-14, about the institution of the Passover, which seems odd. But the real change is the Collect: from "O God, from whom Judas received the punishment of his guilt ..." to "O God, who have called us to participate ... [in] the banquet of his love."
As a pre-emptive strike: Before I am accused of having capitulated to Traddie-land, let me say that, given how much is going on on that day, I would, personally, prefer some of the lost texts from above, the stripping of the altars, and the transposition to the Easter sepulchre. From my limited perspective, the main themes have always been kenosis, abandonment, and desolation, besides the institution of the Eucharist. Hard to integrate all that with happy-clappy. But, as I have already indicated: in this, as in other things, I am out of my depth.
P. S.: I seem to recall a Holy Thursday rite, at this wonderful building -- wonderful ante-wreckovation -- that featured both balloons and female liturgical dance. But, at my advanced age, perhaps I am merely hallucinating. Not much to cry about, given the major losses of that area, however.