1. A donkey's colt was probably the stupidest animal you could pick to represent that you were a king, for this King was 'humble, unassuming.' A zealot would have dismissed Jesus as soon as he saw that. Luke-Acts are material written to demonstrate to Roman officials that Paul was not a revolutionary leader in the category of violent Jewish zealots. Not that the theology of Rome was safe around him, though!
2. Isaiah 55:12. This same chapter shows us that the promise of God is fulfilled in Christ the Servant King (chs 52-53, but earlier as well) and is not void. It is quoted in Acts 13's sermon--that the promises to David were transferred to Christ, as clarified in Acts 2:30-31. The accomplishment of the Servant is what 'makes the nations run to you' (the Servant or "Israel"). Jesus had mentioned this when the Greeks came to find him in Jn 12: I will draw all men to me.
The Creator and The Prayer About His Enemies
Read 4:23--33
In a way, the re-gathering of the apostles after the confrontation with leaders has the most interesting twist so far, as far as early Christian doctrine. The believers remind each other that they serving and appealing to the Creator of the world. This is a line from Ex 20 and Ps 146.
The significance of referring to Ex 20 (the 10 commands) is that the theme of the New Moses has been mentioned, and Christ is seen as the Creator and law-giver. It is his. This matches the declaration of the resurrection, which we could now call the re-declaration: that this world belongs to Christ.
They are not requesting anything. Instead they go to Ps 2, another heavily Messianic psalm. Yet also a realistic psalm because it prepares them (and us) for rejection. Christ is the rejected owner.
This is not much different from the parable of the tenants of Mt 21. "Let's seize the heir and then the vineyard will ours." So this is how the apostles view the situation they have come to. They have made 3 utterly clear statements about what Israel is supposed to be doing in God's "vineyard" and it is being refused. The parable of the tenants had declared that if this happened, the vineyard would be given to another people. While this is the term 'ethnos,' it is not simply the Gentiles; Jesus meant anyone who believed and produced.
We should comment here on something often declared about history, about the treatment of the Jews. There was a crass belief in church history that the Jews suffered all through history because they took the life of Christ. This is close, but leaves something ambiguous that needs, as we say in modern times 'disambiguification.' The problem is actually the refusal of the mission work of Christ.
If we drop to an upcoming verse (27), we see that a powerful sovereign force of God's (his hand and will), caused the death of Christ to happen through the leaders and people. The half-Jew Herod is named, and Pilate, and "the Gentiles" (to cover all of Rome) and the people of Israel. Can't see where they missed anyone.
So that much is on the 'hand and will' of God. But now Israel (and anyone who 'sees') is supposed to join the thousands of believers in Christ's mission. To review, this had been the appeal of Peter just a short time ago, with the warning: anyone who does not follow the New Moses will be dramatically disenfranchised from God's people. The apostles made many warnings about the upcoming destruction of the nation, Paul doing so through Luke his transcriber, but also in his letters, in Thessalonians, and warnings in Hebrews.
The other historical mistake is that this should be a permanent condition. That is totally wrongheaded. After the destruction of the nation, when the apostles remaining simply continue on in outreach, Jews were welcome. Oddly, some Jews doubled-down for further revolt! Martin Luther, before his sickly late condition, sounded like Romans 11 about the Jews: they could be great evangelists.
The Psalm 2 has as broad a warning as Ps 110 above. Yes, the leaders gathered against the Lord and His Christ (this language was just used by Peter to finish his first speech; Lord and Christ). But they are further warned by the psalm. It is extensive, vs 4-5, 9-12. The Lord will smash the enemies of the Son. He is given a rod of iron, which suggests an overwhelming bludgeoning rather than one on one combat with a sword.
Between warnings we should recall the 'you are my Son, this day I have installed you as King.' We should note that this psalm could have been an alternate to 110 for the declaration that the King is enthroned. It's the same. He is enthroned, and all nations, leaders and the average person must submit to him. "Kiss" is meant in the royal sense, that display of subjection.
But the prayer of the apostles is for boldness for further speech, and to ask the "hand" of God (the one just mentioned which brought about the Gospel event) to further support what they are saying by healings, signs and wonders in Christ's name. As you may have noticed, other than the Spirit's languages, there has only been one other 'miracle.'
While the apostles are opposed to the leaders of their time on many points, they do not plan to be in their faces about it. They get a 'sign' alright: the rattling of the place where they are gathered, which may possibly have been a corner of the temple yards. They were definitely given the ability to continue to speak further what they were saying. All of them became prophets declaring the warnings of Ps 2 etc.