According to Catholicism, why does the Creed mention only the role of Holy Spirit in the conception of Jesus? We read an account of the Annunciation at Lk 1:30-35 (NRSVCE):

30 &quot;The angel said to her, “Do not be afraid, Mary, for you have found favor with God. 31 And now, you will conceive in your womb and bear a son, and you will name him Jesus. 32 He will be great, and will be called the Son of the Most High, and the Lord God will give to him the throne of his ancestor David. 33 He will reign over the house of Jacob forever, and of his kingdom there will be no end.” 34 Mary said to the angel, “How can this be, since I am a virgin?” 35 The angel said to her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be holy; he will be called Son of God

We see the reference to the Most High being made twice - first at Verse 32 and secondly at Verse 35. Throughout  the Gospels, we see Jesus witnessing, communicating with and praying to God the Father whom he fondly calls Abba Father (Mk 14:36). That implies that by the term the Most High, the angel meant God the Father. Verse 35 also points to the role of the Most High in the promised conception of Jesus . Now, the Creed says that Jesus was conceived by the Holy Spirit and born of the Virgin Mary, apparently ignoring the role of the Most High. One wonders why the Church limits the conception of Jesus to the power of the Holy Spirit in the Creed . My question therefore, is:  Why does the Creed mention only the role of Holy Spirit in the conception of Jesus ? What explanation does the Catholic Church offer for the same?
On your first question: the text (Luke 1:35) says that the pregnancy of Mary is by the power of the Holy Spirit, but that behind his work (&quot;overshadowing&quot;) is the work of God the Father. So I would suggest that this means the initiative is from God the Father, but the Holy Spirit is the one who accomplishes the work. (Who can explain the inner workings of the Trinity?)
Compare with what the angel says to Joseph in Matthew 1:20:

&quot;Joseph son of David, do not be afraid to take Mary home as your wife, because what is conceived in her is from the Holy Spirit.&quot;

No mention in this verse of God the Father - yet of course that does not mean he was absent.
Saying, as the creeds do, that Christ was conceived by the Holy Spirit, does not in any way diminish the role of the Father. I think they are just affirming that Jesus was conceived not by a human Father but by the agency of the Holy Spirit.
On your second question - I do not have expertise in Catholicism, so perhaps someone else could answer from that angle.