Adopted as Son of God at the Baptism?

Anyone remember the 1999 tv special on Jesus that is now available on youtube – this clip combines Jesus and John as relatives (Lk 1:36, + childhood story from Lk 2:42-52), the public nature of the Spirit’s descent and divine voice (Matt 3:16-17/Lk 3:21-22) and “lamb of god” from John 1:36 (and the following temptation scene is pretty entertaining and a little bit of a confused take on what Christians have believed about the nature of the Incarnation and hypostatic union).  Aside from the typical harmonzing of the movie, there is much to discuss about the episode in Mark and I have tried to go through some of the commentaries:

• John the Baptist appears on the scene as the “voice in the wilderness” (cf. 1QS 8.12-16 for the Qumran communities self-understanding of their role in the wilderness) and forerunner of the Lord.  Mark later explicitly identifies him with Elijah (9:11-13), but there is already an echo in the choice of John’s attire (2 Kings 1:8; a full study of John’s diet has been done by James A. Kelhoffer, The Diet of John the Baptist).  It is also worth comparing the similarities and differences from Josephus’ characterization of the Baptist (Ant. 8.5.2) and the continuities/discontinuities of John’s innovation of a single baptism with other ritual purification washings or the later practice of Gentile proselyte baptism and the meaning of John’s rite.  The location in the wilderness and Jordan River evoke ideas of a new exodus and entry into the Promised Land and John may be calling for national repentance (see Brian S. suggestion in the comments that Jesus identifies with this project) before the coming eschatological agent who is likely a human figure (cf. John unworthy to untie his sandals).
• Notice the changes in the Synoptic parallels to Mark 1:10-11.  Mark has a private vision that “he [Jesus?] saw” (εἶδεν) and describes it in violent imagery with the heavens being torn open (σχιζομένους) (cf. Isa 63:19LXX; see also Mk 1:13 the spirit “drives out” [ἐκβάλλει] Jesus into the wilderness) and the spirit descends as a dove (contrary to a gentle image see Maurice Casey’s analogy of a flapping pigeon) “into” (εἰς) Jesus followed by the divine voice.  Mark’s scene is of a divine invasion!  This is softened in Matt 3:16-17/Lk 3:21-22 as the heavens open (ἀνοίγω in different moods) and the spirit descends on (ἐπί ) him and Matthew also changes “you are” (σὺ εἶ ) to “this is” (οὗτός ἐστιν) my beloved son.
• Does Mark indicate that Jesus was adopted as son of god at the baptism (yet note the quotation of Ps 2:7 stops short of quoting the line “today I have begotten you”), reflecting an adoptionist christology that figures like Justin Martyr, Irenaeus, Eusebius 3.27; 5.28, etc, opposed?  If one goes through some of the more traditional commentaries one can find debates about whether Jesus became the Son at the baptism or if the voice rather declares what Jesus already was, but in my view I wonder if we ought to be cautious in reading later christological debates about divine sonship into Mark’s account here.  What if the reference to “my son” is the Davidic king of the royal Psalm then the story can be read as his annointing for messianic office for which he was enthroned at his exaltation (cf. Mk 12:35-37) and possessed by the divine spirit (cf. 1 Sam 10:10-12; Isa 61:1, cf. the eschatological advent of the spirit)?
• As for the intertextual echoes in the divine voice, there is most likely an allusion to Psalm 2:7 on the Davidic king as god’s son, possibly to Isaiah 42:1 on the servant and a slight chance that there is an allusion to the Akedah or binding of Isaac is the reference to the “beloved” son (I am not so sure that we should read so much into a parallel based on a single word).

What do you think was the purpose of Mark’s account of the baptism by John?