Why did the Pope even mention same sex couples then?
I don't know the answer to that, but the "Presentation" section of the document appears to give some reasons for the document itself:
This Declaration considers several questions that have come to this Dicastery in recent years. In preparing the document, the Dicastery, as is its practice, consulted experts, undertook a careful drafting process, and discussed the text in the Congresso of the Doctrinal Section of the Dicastery. During that time, the document was discussed with the Holy Father. Finally, the text of the Declaration was submitted to the Holy Father for his review, and he approved it with his signature.
While the subject matter of this document was being studied, the Holy Father’s response to the Dubia of some Cardinals was made known. That response provided important clarifications for this reflection and represents a decisive element for the work of the Dicastery. Since “the Roman Curia is primarily an instrument at the service of the successor of Peter” (Ap. Const. Praedicate Evangelium, II, 1), our work must foster, along with an understanding of the Church’s perennial doctrine, the reception of the Holy Father’s teaching.
As with the Holy Father’s above-mentioned response to the Dubia of two Cardinals, this Declaration remains firm on the traditional doctrine of the Church about marriage, not allowing any type of liturgical rite or blessing similar to a liturgical rite that can create confusion. The value of this document, however, is that it offers a specific and innovative contribution to the pastoral meaning of blessings, permitting a broadening and enrichment of the classical understanding of blessings, which is closely linked to a liturgical perspective. Such theological reflection, based on the pastoral vision of Pope Francis, implies a real development from what has been said about blessings in the Magisterium and the official texts of the Church. This explains why this text has taken on the typology of a “Declaration.”
Is the couple supposed to announce that ahead of time?
Are they supposed to announce that they are gay? Not that I am aware of.
Why not bless one person at a time, instead of them as a "couple"?
I suppose you could do that. I mean, if me and my younger brother are taking a Holy Land tour given by the Franciscan Friars, I image that we might approach the priest together at some point and say "Father, could you bless us." I don't think he has to say "OK, please separate, I will bless you IcyChain and then I will bless your brother." It is not like he is blessing our union as "brothers" when we ask for a blessing at the same time.
I think the document contemplates situations like that:
40. Such a blessing may instead find its place in other contexts, such as a visit to a shrine, a meeting with a priest, a prayer recited in a group, or during a pilgrimage. Indeed, through these blessings that are given not through the ritual forms proper to the liturgy but as an expression of the Church’s maternal heart—similar to those that emanate from the core of popular piety—there is no intention to legitimize anything, but rather to open one’s life to God, to ask for his help to live better, and also to invoke the Holy Spirit so that the values of the Gospel may be lived with greater faithfulness.