Solas-related thought happened in my head today.
Maybe someone else already thought about this, but...
I think Solas took the eluvians back while we were at Halamshiral.
Everyone seems to basically think his agents got the password for him, or that he wrested it away by force. Well, he might have sent agents to clean the eluvians out by force, but he didn't have to use any sort of power to take them.
Remember how he was being all casual there and playing the servant? Even servants have their hierarchies, he tells us... while off all to himself, socializing with the elves. It was the perfect opportunity for him to gain enough trust to get the eluvian network password. And, as someone "in" with the Inquisitor, all the better--they'd have thought he had access to resources and people that they needed for their uprising. Solas himself got the password. Then he sent agents in to do his dirty work. He wasn't just giddy because he was drunk, he was giddy because he'd participated in some of the danger and intrigue; the Inquisitor was completely unaware of him having done anything but listen to the servants' gossip. I suppose he wanted to end the night with a bit of victory sex with Romanced-Lavellan, assuming she was willing.
Of course, I might be 100% wrong. He might have continued to send agents in before he managed to get a foothold. But, I would not be surprised at all if Solas's agents had been responsible for some of the elves who died that night.
Personally I don't think this happened at all, and to be honest I'm not sure why there are quite a few people I saw who think that this is what happened. Because both him and Briala are there? Because he's excited in Winter Palace, when it's been established in the story that Solas enjoys court intrigue and a visit at the Winter Palace reminded him of how he enjoys it? This is some flimsy evidence.
After all, depending on who Inquisitor chose to be in their combat party (I assume all companions and advisers are in Halamshiral either way, judging from later banter) Solas has to be at Inquisitor's beck and call, and while he's definitely having fun in Winter Palace, he has to be very careful not to out himself.
So no, I don't think this is when he took Briala's eluvians.
When do I think eluvian retaking began?
In Temple Of Mythal.
After all, look what surrounds the Well of Sorrows?

Eluvians. Not just one, but several of them. This is basically an eluvian hub.
And remember how Solas tells us "She (Morrigan) is right about only one thing - we must take the power that lies within that Well".
So he's reluctant (either because he doesn't trust Morrigan or Inquisitor, or worries about Inky), but still advises to take it. Why, I wondered?
Well, I think I know the answer now. After all, what happens with the Well after it's been emptied?
A mysterious watery lady rises from the basin and collides with the eluvian. Interestingly enough, despite Inquisitor and the rest landing directly in Skyhold (they don't go through Crossroads, the order which they appear in Skyhold and the speed with which they do it mirrors the order and speed they've entered ToM eluvian) the water lady doesn't follow, possibly because she stayed IN the eluvian.
Basically, I think this is when the eluvian network got re-opened. There are no longer just a few eluvians "accidentally left ajar" (as Morrigan tells us) that must be pried open through knowledge, power or keys, but pretty much the entire network got itself revitalized.
We can see the effect of it if we have OGB world-state - the eluvian in Skyhold suddenly gets opened; and it gets itself opened into raw Fade - a feat Morrigan claims requires immense power. And I don't think this is solely because of Flemeths' or Kieran's power. I think the feat could be accomplished only because the eluvian network itself is buzzing with new energy.
It would also be another good reason for Solas to go after Flemythal and steal her power. After all, he did try to re-acquire access to eluvian network in The Masked Empire and killed Felassan for failing to do so and seeing how much the network is featured in Trespasser indicated that re-acquiring eluvians is absolutely crucial for Solas' plans to work.
Then, of course, is this.

People usually interpret this moment as Flemeth "sending something through eluvian". I don't think this is that at all. Judging from past encounters and her pleased smile (after meeting with Inky and Morrigan in the Fade) Flemeth has long made contingency plans, so whatever she has in store for Morrigan or whoever else, she's likely sent or stored her energy away a long time ago. What she does here, IMO, is either revitalizing another part of eluvian network OR preparing herself to override Briala's password (or luring Solas by appearing to do so).
In other words, I don't think this is a coincidence at all that they meet under the eluvian or that one of the last scenes in the main game is this:

I think this is a moment when Solas not only took Mythal's power, but exactly when he took over the eluvian network.
Also... this eluvian... it's quite big, isn't it? Pretty much as big as the eluvian we see behind Solas (and to which he disappears into) in Trespasser. I don't think this is coincidence at all as well. I think that all the small hints and visual storytelling make it a far stronger case as to when exactly Solas gained eluvians for himself compared to Halamshiral.