The Crucible is explained to be a power source, it does not control or effect the Catalyst in other ways. You're speculating that the Catalyst is unable to take action, but nothing that's said or seen in game tells us that is true. The Catalyst was designed to find a solution to the synthetic problem and to oversee synthetic-organic relations on it's own terms. It isn't shackled by anything except what is was programmed to do and it mentions it controls the Reapers.
The Catalyst places organic life over synthetic life as that's what the Leviathans were concerned with. Their slaves were dying, they wanted to stop that. If the Catalyst had monitored synthetic development closely, then the risk of synthetics overpowering their masters would be minimal. Along with developing countermeasures against potential synthetic uprisings it would have been able to put down those uprisings immediately. Watching over synthetics shouldn't be a problem for the AI. It has no personal feelings to stop it from doing its work, I don't see why watching over development of synthetics would even be an issue. There's really no way to make the Catalyst's solution seem anywhere near logically sound. The synthetic problem isn't as simple as organics build synthetics therefore organics are the cause of potential synthetic conflict. There are little variables in the relationship between man and machine that the AI just isn't looking at.
I seem to remember the Catalyst explaining it to Shepard when talking about Control. That the Catalyst does not look forward to be replaced by Shepard, but that both it and Shepard are bound by these choices or something and that the Catalyst is forced to accept it. Might need to recheck that dialogue, but I think I took it that way that the Catalyst cannot do anything. Why else would it be hostile if only Destroy or Control are available? It doesn't want these solutions, yet it has to accept them somehow.
Also when it says it cannot act on any of the solutions... doesn't that mean that it... well, can't take action?
Well, maybe the mistake is we never know what it has tried, witnessed and studied over all these years, but to me this is not a big issue. It chose a solution which it thought best. And yes, I disagree with it, that's why I like Destroy best.
Synthesis is about elevating organic/synthetic life by combining synthetics and organics. Whether or not people are working together during the time of the Reaper invasion doesn't have much to do with them being ready to accept synthetic technology into their bodies. That is what the synthesis ending is about, Synthetics accepting organic life into their whatever and organics accepting synthetic tech into their bodies. So the idea that a cycle is ready to undergo synthesis because they're working together doesn't really make sense. The huge issue with this is what makes an organic/synthetic ready to undergo such a huge transformation? It's most likely related to biology or design of the synthetics, so what makes humanity, asari, turians, etc different from say the Protheans? The Protheans were arguably superior to any races alive in the current cycle, why weren't they ready? What about the Leviathans? They're pretty much perfection incarnate, what deemed them not ready?
And if it was a mental thing that determined whether a cycle was ready to accept synthesis, then wouldn't there be a large amount of individuals who might be opposed to becoming part synthetic? There's no way to understand what the Catalyst means when it says "other civilizations weren't ready".
The idea in general that waves of energy could somehow incorporate synthetic technology into organics and organic life into synthetics is ridiculous by itself anyways. It's easier to accept the idea that energy blasts could help Shepard control the Reapers or wipe out synthetic life in the case of the Destroy ending.
Yeah, maybe it doesn't make sense, it's just speculation because we don't know what the exact reason is.
You don't need to argue with me about what happens during Synthesis. I chose it the first time, unfortunately, it wasn't for me. But that doesn't mean that other people can't enjoy it. And they can make up their reasons why this cycle was ready and what will happen next.
Yes, when you choose Control the Catalyst gives you the ability to do whatever with the Reapers. But that's the exact issue, what if you choose to wipe out organic life? You are not bound by programming like the Catalyst is, insane Renegade Shepard could very well do that.
Aside from that though, the idea that pushing the responsibility of a problem to someone else is a solution is just wrong. Okay, Shepard has control of the Reapers now. How is organic life saved? Is Shepard supposed to rule with better judgment than the ancient AI with the knowledge of countless civilizations at its virtual hands? Is the Catalyst acknowledging its own insufficiency by pushing the responsibility of the synthetic threat onto Shepard. Either way, the control ending certainly isn't a solution to anything.
Key word "could". This is up to each player. Headcanon your ending in a way in which you're comfortable with it. Easy as that.
I agree Control isn't really a solution to anything, but Shepard can act differently depending on what each player will think of. I think we see the Reapers helping rebuild, which is already a step in this direction, they help instead of continuing the harvest.
Like I said, I'm not a fan of Control, or Synthesis for that matter, but I always try to roleplay and maybe one of my Shepard's think one of those is the best solution (while we as players know the outcome and what's going to happen, Shepard doesn't).
The destruction of Synthetics does NOT come with the Crucible, it is a power source. The Catalyst offers you this solution, the Crucible cannot. What even separates Synthetics from other technology? Is the energy blast shot off by the Citadel an EMP? Why does it cause only minor damage to some electronics and completely wipe out synthetics? It just doesn't make sense.
I can only imagine how future synthetics will feel if they learned Shepard decided to wipe out all synthetic life in the galaxy just to destroy the Reapers. Not going to make them feel very equal or accepted.
What about the sentence "the Crucible will not discriminate"? Here you have the most important sentence, the energy of the Crucible will target synthetics.
A common interpretation here is that it only targets Reaper tech though, which makes the most sense, given that clearly not all tech is affected. But everything with Reaper tech is gone. Reapers, EDI, Geth.
I think you're not too far off with the EMP, because I think I remember one of the writers stating somewhere that the relays were supposed to just overload (post EC iirc).
And once again, these are the Catalyst's choices. The Crucible has no mind of its own, it is a power source. The Catalyst is an advanced AI capable of making its own decisions and the Crucible was developed later (somehow, I have no clue) by another cycle. Here's the odd thing about the Crucible and the Catalyst though. If the AI had tried to use synthesis before, it would have needed the Crucible to do so. So that means a civilization would have had to attach the Crucible to the Citadel and get as far as Shepard to make that happen. Now, does that mean Shepard isn't the first one to stand in front of the Catalyst? Or did the Catalyst just get some juice hooked up to it and decided to blow some green energy over the galaxy? Confusing.
The Crucible was initially built as a weapon against the Reapers without the Citadel. The Citadel was only later incorporated into this idea. I think because of its star-shaped form when open, the energy can be spread much further (if the ancient civilizations knew that or not, we don't know). So if the Crucible is a weapon, somehow the solution for Destroy would need to be already present on the device itself. Otherwise it makes no sense.
Maybe some argued to control the Reapers, the design evolved over many cycles, so it could've been added at some point as well. Javik says the Protheans also believed that the Crucible could be used to control the Reapers.
I think the Catalyst didn't know about the Crucible being able to be efficient for Synthesis. It doesn't even know before the Crucible docks, that's why the Reapers still attack it to that point. It only knows it's supposedly some kind of weapon, and wants it gone.
And we don't know what the Catalyst tried as a Synthesis solution, could be anything from merging the two together physically to different other things. It's not really relevant anyway.