"is the ABC cycle for the propers mandatory?"

There is a rubric in the "ordo cantus missae" that allows for pastoral reasons that any Latin chant from a given part of the mass can be exchanged for another chant from the same part that is from the same season. In other words, it is possible to sing, for example, the "rorate caeli" (which is the introit in the Roman gradual on the fourth Sunday of Advent) on every Sunday of Advent. Or, another example, "In voluntate tua" (for the 27th Sunday in Ordinary Time) on every Sunday in ordinary time. If this is vallid in latin, I think it is vallid in english adaptations of the proper.

Therefore, if anyone wants to change and choose which introit, offertory, communion (and even gradual or alleluia) they prefer to sing for pastoral reasons (for example: to "maintain a unified theme" with the lectionary readings) this is permitted by current ecclesiastical legislation on Ordinary form masses.