Well, of course you can. But to plan to do it without regard for other divers is just rude.
I don't do swim-throughs any more. I advise the DM and the other divers and I stay out of the way as other divers enter the swim-through. Planning to swim up to the entry, take a peek, and make the decision is likely to unnecessarily screw up other divers.
I have watched divers decide at the last second to not do a swim-through. By the time they made up their mind and backed out, the following divers were no longer able to find the line of divers who had entered. It sometimes happens with inexperienced divers. But if a diver is going to plan that last second decision, they need to let other know before the dive and then be last going in. It is just consideration for others.
How would it screw up other divers? And what's to find? It's a swim through, not a maze... Sure, it's courteous to not block the entry while you're deciding. But I habitually hang at the rear of any group, and it has never bothered me in the slightest to wait while someone peeks before they go in. It does bug me a little when the people in front scissor kick their way through and silt it up, but there's enough flow in most that this isn't a huge issue. I just leave a gap between me and the people in front of me.
I just don't really see this as being a problem.
I have not been on this dive so I am only going off the story, but 20mins in is no longer a swim through that is an overhead environment. There are many buzz words that make me cringe when a diver says it was "tight" or looking into "blackness". This means divers are going to places they shouldn't be. There is a reason recreational diving and technical diving are separate, part if it is training and equipment the other part is comfort in the water. Again I have not been on this dive but from the description it sounds like the DM should not have been taking any one in there 20min is not a swim through. You made the right call if you weren't comfortable then turn the dive around. Yeah SMB and light are important to have but to do overhead environments you need more than that. Every year people get over there head in overhead environments get lost or panic and drown because they didn't realize the danger they can hold.
The Devils Throat is not particularly tight, as evidenced by the OPs ability to turn around in it, and swim past other divers to get back to the entrance. Nor is it black. Dark, yes, but it's not black. Lots of people don't even use lights, although I do, simply because I don't see the point of going in if I can't clearly see the stuff inside. If I wanted to dive blind, I'd stop using a prescription mask... But lights (while being a good idea) are not required for finding your way through.
Being in a near-panic state will absolutely affect judgement, and no doubt contributes to the description being given.
And you'd have to be going
really slow to spend 20 minutes in the Throat. While I've never measured it, I'd say the tunnel is maybe 60 feet end to end. As you can see in this video, it is not particularly tight. And although the diver is using a light, there are quite a few openings to let in light.
[video=youtube;8ECFvuLBUNA]https://www.youtube.com/watch?v=8ECFvuLBUNA[/video]