1. How does Ganondorf obtain the Triforce of Power in Twilight Princess during the execution scene?
2. When Princess Zelda sends Link back to his youth at the end of Ocarina of Time, what happens to the Triforce of Courage? I think that Young Link possesses it when he returns to his past, so what happens to it in the Adult Era of the timeline? Is it missing? If not, where did it come from? It obviously reappears sometime in the future because we are tasked with finding it in The Wind Waker.
These are both related. I thought HH answered these, but looking at it now, it's a little vague.
Link bears the proof of his courage on the back of his hand. Because Ganondorf was sealed away along with the Triforce of Power, the rest of the Triforce remained with the chosen ones. - HH, p. 110
It's odd that they mention Ganondorf being sealed, since that happens on the AT, not the CT which this passage is about. I think it might be a mistake, or at least an oversimplification. Either way, we know one thing for sure, and can infer another. Link carries the crest of the ToC when he returns to the past. This is shown in-game and stated accurately a few times in HH. Second, the CT Triforce was forced to separate to the chosen ones without anyone entering the SR and touching it.
The Ganondorf of this timeline is able to wield magic due to his possession of the Triforce of Power. However, since Link was able to warn Princess Zelda of the future, Ganondorf did not enter the Sacred Realm or lay his hands on the completed Triforce. Link returning from the future bearing the Triforce of Courage made it so that Ganondorf was unable to consolidate the omnipotent power of the Triforce within himself. - HH, p. 113
This passage doesn't make much sense either. Why wouldn't Ganondorf be able to consolidate the Triforce? Wouldn't he simply need to bring the chosen ones together like he did twice in the AT? This one does say that Link actually returned with the ToC itself, not just its crest. I suppose in a roundabout way it's trying to say that this caused the Triforce to be split, granting Ganondorf the ToP (before he was able to enter the SR and try to take the whole thing). So that answers your first question.
The second question is answered in WW. The HoT departing from the timeline caused the ToC to break apart and spread across Hyrule. That's why WW-Link has to salvage the pieces. Of course, there's the awkward notion of the ToC remaining on the AT while still being carried to the CT. However, there are not two copies of it on the CT. The one he 'brought from the AT' is actually the CT version. What he really carried was the
proof of courage, causing the CT Triforce to instantly split and fall into the hands of its rightful owners.
3. What is the Dark World in Four Swords Adventures? Are the Dark Work and the Dark Mirror the Twilight Realm and the Mirror of Twilight?
It's been a very long time since this topic has been addressed. It requires digging into the Japanese names to get a straight answer. If I remember correctly, they are not the same, though there was a lot of debate surrounding it. However, this was many years ago. Frankly FSA's position on the new timeline shows me a possible intention of connecting the two. The only barrier to this would be the fact that that Mirror of Twilight was destroyed, though it's possible that there is more than one mirror.
4. I noticed in Hyrule Historia that the section on Four Swords Adventures says that the Light Force used to rest within the Four Sword. But as I understand it, the Light Force used to rest within Princess Zelda, not the Four Sword. What’s up with that?
Just ignore everything HH says about FSA. Most of it is wrong. It also says that Link released Ganon (instead of Vaati) from the FS at the beginning of the game, for example. In this case, it's just Force that used to rest within the Four Sword, not
the Light Force specifically. (unless, of course, they're trying to retcon that)
This is a notable oddity though. At the same time that HH seems to be pushing a stronger, more centralized concept of "force", SS (at least partially) retconned the source of Zelda's power, initially described in MC to be the LF. So, what is it? Hylia or the Light Force? What exactly is the Light Force? Is it related to Hylia? I think they'll be expanding on the Light Force in the future.