Good post, Cary. As dmatic so often does, he assumes a much more expansive view of a word or phrase than what was intended in the Biblical text. You have concentrated on the "restitution" part but I would like to take a look at the meaning of "all things".

Here is Strong's definition of the Greek word which is translated as "all things" in Acts 3:21:

all, any, every, the whole:—all (manner of, means) alway (-s), any (one), X daily, + ever, every (one, way), as many as, + no (-thing), X throughly, whatsoever, whole, whosoever.
As you can see from the definition it essentially means "all" but does that mean that every time the word is used it must imply absolutely everything, as in the entire universe? As with many definitions, it doesn't always imply an absolute but can be limited in its meaning and is often limited by its context. For example, in the following verses the same word is used:

1John 2:20 But you have an unction from the Holy One, and you know all things.
Does that verse mean that everyone who is indwelled by the Holy Spirit knows absolutely everything, even everything that God knows? No, it obviously means that the Holy Spirit supplies us with everything we need to know, especially spiritual things.

2Cor 6:10 As sorrowful, yet always rejoicing; as poor, yet making many rich; as having nothing, and yet possessing all things.
Did Paul mean that he actually possessed everything that exists?

So we can see that "all things" doesn't always imply everything in the universe. So what does the "all things" mean in Acts 3:21?

Acts 3:21 Whom the heaven must receive until the times of restitution of all things, which God has spoken by the mouth of all his holy prophets since the world began.
The first thing to note is that the "all things" are qualified by the verse. They are what was spoken by the prophets, and from Mark 9:12 we know that it is mainly talking about what is mentioned in Mal 4:5-6 regarding the coming of "Elijah":

Mal 4:5 “Behold, I will send you Elijah the prophet before the great and awesome day of the LORD comes.
Mal 4:6 And he will turn the hearts of fathers to their children and the hearts of children to their fathers, lest I come and strike the land with a decree of utter destruction.”
These verses are talking about the Day of the Lord, which includes the Great Tribulation and the millennial reign of Christ which follows His second coming. So the restitution, or restoration, of all things means the judgment of the wicked, the saving of "all Israel" and the millennial kingdom of Christ when Israel will be restored to their land and blessed by God. It does NOT mean or imply that in the end everyone will be saved. And that fact can clearly be seen in the verses just prior to Mal 4:5-6:

Mal 4:1 ​“For behold, the day is coming, burning like an oven, when all the arrogant and all evildoers will be stubble. The day that is coming shall set them ablaze, says the LORD of hosts, so that it will leave them neither root nor branch.
Mal 4:2 But for you who fear my name, the sun of righteousness shall rise with healing in its wings. You shall go out leaping like calves from the stall.
Mal 4:3 And you shall tread down the wicked, for they will be ashes under the soles of your feet, on the day when I act, says the LORD of hosts.
The wicked becoming burnt stubble without root or branches and becoming ashes cannot be talking about universal salvation.