The fact that the commandments finish with the people complaining to Moses implies he has not yet gone back up?
So do I understand correctly, the ten commandments are delivered by Yahweh on the mountain, while Moses is still at the bottom of the mountain?
Where is "between the LORD and you[the people]"?
First let's address the second question in case the answer may change our understanding of #1 or #3, depending whether you believe that this is Moses speaking the words God just spoke to him on the mountain, or whether you believe God is speaking the commandments directly to the people.
Version #1 - Moses is just the messenger
So Moses went down to the people and told them. "And God spoke all these words, saying,..." (ESV Quotes added by me)
If we read from Ex. 19:25 straight through to 20:1 you might conclude that it is part of what "Moses told them" (19:25) and 20:1 is just Moses speaking.
So Moses tells them the ten commandments that God told them, just as he has repeated many messages before and after this that God told him in private.
In this case, Moses would have to be with the people, and would most likely be at the front. One could imagine the natural "front" of the crowd would be facing the mountain, delivering the message he received from and about the God that is currently covering said mountain, and thereby "between" God and the people.
It could also be that God begins to speak and the people cannot bear it. Deut 5:4-5 could read that Moses stood between them in order to declare the word of the LORD because they were afraid of the fire. And it is Moses speaking when 5:5 says "He said", telling the people what God told him.
Version #2 - God spoke to all Israel
God gave Moses instructions in 19:21-24 and in v25 Moses goes down the mountain to tell the Israelites (them), reminding them to stay off the mountain.
Now that the people are ready, God himself speaks to the people (20:1 "God Spoke"). As they are listening, they see the cloud and the storm and are so frightened they ask Moses to speak to God for them.
Moses has not had time to go back up, with or without Aaron, as he is still with the people. It is still easy to imagine Moses being right in front, either having just come down, or simply being in front of the people as their leader.
Moses is with the people
So it seems, either way, Moses is with the people immediately after the giving of the ten commandments.
Does Moses go back up in 20:21?
It is my opinion that Ex. 20:21 is not a full ascent and descent, but rather God speaking to Moses through the cloud, possibly at the foot of the mountain which the cloud is covering. I believe this because the verbiage changes. Moses "draws near" in 20:21 instead of "going up" ('alah). Also, he then goes to tell the people what he has been told in 24:3 but it is not "go down" yarad but simply a basic verb for come/go/enter ([bow']) with no directional reference (except maybe "in"), be it up or down. Also this clarifies the confusion over the command to bring Aaron next time Moses goes up. If this is not an ascent then we see that the next time Moses goes up in Ex.24:9, he does indeed bring Aaron and so he is faithful to God's command.
What does he mean by Between?
We should not assume that "between" in Deut 5:5 means only Moses' physical position , but perhaps is also describing his relational position, his role.
Moses was a mediator between God and the Israelites throughout their time at Mt. Sinai and continuing in the wilderness. Later in Moses' monologue in Deut. 10:10, he reminds them that he interceded for the people and the LORD did not destroy them. So maybe he is likewise reminding them in Deut 5:5 how he stepped in and spoke to God for them, assuming a mediating position between them.
Galatians 3:19 does not name Moses as the mediator, but most commentaries (see link for some) understand the reference of the giving of the law as being Moses.
- Moses is still with the people, not going up with Aaron yet
- Yahweh delivers the commandments with Moses with the people.
- Moses has taken a physical position between God on the mountain and the Israelites, but he has also taken a relational position between God and man. He is mediating between God in heaven and on the mountain and the people below, communicating and interceding.