The Immaculate Conception is not a Holy Day of Obligation in Australia.
…Is this the answer to my question? The mass of the day will be used, with nuptial blessings. Does this just mean that the readings and other proper prayers will be the prayers for the mass of the solemnity, and the mass will exactly resemble the mass of that solemnity, except the nuptial blessings will be tacked on?
Yes, I think so, although the readings is complicated. The Rite of Marriage will be after the homily. The Nuptial Blessing will be after the Our Father.
For example in the Prayer Over the Gifts instead of being able to have from Wedding Mass C:
“Lord, hear our prayers and accept the gifts we offer for Steve and Jane. Today you have made them one in the sacrament of marriage. …”.
Instead the priest is required to have:
“Lord, accept this sacrifice on the feast of the sinless Virgin Mary. You kept her free from sin from the first moment of her life. Help us by her prayers, and free us from our sins. …”
Similarly for the entrance antiphon, opening prayer, preface, communion antiphon, and prayer after communion.
I think one reading can be changed. It has in the Rite of Marriage:
“11. … The liturgy of the word as adapted to the marriage celebration, however, is a highly effective means for the catechesis on the sacrament of marriage and its duties. Therefore when the wedding Mass may not be held, one of the readings from the texts provided for the marriage celebration (nos. 67-105) may be chosen except from Holy Thursday to Easter, on the solemnities of Epiphany, Ascension, Pentecost, or Corpus Christi, or on holydays of obligation. …”. (The Rites Volume One, Liturgical Press, 1990, ISBN: 0-8146-6015-0, page 722).
Since it is not a holyday of obligation in Australia it would seem reasonable for the priest to allow one reading to be changed.
For this solmenity the Gloria is said (even though it is in Advent, see Missale Romanum, Libreria Editrice Vaticana, 2002, ISBN: 8820972719, page 878). So is the creed.
The advent restrictions are, from the 2002 General Introduction to the Roman Missal (GIRM) which can be accessed from
romanrite.com/girm.html :
“313. … In Advent the organ and other musical instruments should be used with a moderation that is consistent with the season’s character and does not anticipate the full joy of the Nativity of the Lord.”
"305. … During Advent the floral decoration of the altar should be marked by a moderation suited to the character of this season, without expressing prematurely the full joy of the Nativity of the Lord. "