> Would not a sonar help?

I'm not even sure what you're envisioning but something lightweight and not bulky that is capable of producing sonar images similar to whatever the detectors were in Aliens doesn't exist. And it was zero viz and in completely zero viz you can barely see your hand in front of your mask, assuming you have illumination.

> A full spacesuit-like helmet with air to be able to talk via radio?

That creates a dead space that fills up with CO2, you'd need to actively pump that out through the rebreather which now creates more things to bash into rock and fail and kill you, plus generates more bulk which is the last thing you want into those tight caves.

You can use a full face mask (FFM) that has comms with a rebreather but in a single file cave there's no point and cave divers know how to communicate in single file in zero viz by touch contact. And there's not much to communicate since either you're moving or you're temporarily stuck and the person behind you can't really help you much. With a rebreather you can also talk pretty well in water if you're just close enough to each other. I've also managed to communicate effectively by talking into a normal scuba regulator as well, without having to write anything down.

> Laying down some flexible tunnels that could be inflated with air or maybe just pumped with clear water to gain visibility?

That would probably take weeks and you'd be fighting the pressure of the incoming water inside of the cave. Air probably wouldn't work at all, you'd need to build a pressure differential airlock so that's likely impossibly complicated. Water might work if you built a reservoir at a higher altitude to gain pressure, but them you need to pump the water up there and you need flexible inflatable tubing that can hold a couple atmospheres of pressure and is large enough for a human, and I think I'd prefer to be surrounded by rock rather than by saran wrap that could fail and I'd get entangled it in the shredded plastic and drown. No thanks. As evidenced by the successful recovery, zero viz can be dealt with.

> So many ideas come to mind, but they chose the most difficult transport of sedated and tied up kids ever. I assume they had a good reason, but still do not understand it.

They're all trained at more or less doing that, they've been through the environment, and they've all been through rescue training to drag an unconscious buddy out of the cave. Strapping the kids down, giving them sedatives and putting a FFM on them solved all the problems of taking someone out of the cave that wasn't trained in diving at all. It gives you the smallest package to drag out of the cave along with giving you access to the kids. The coffin that Musk was proposing was more or less just that. By trying to keep the kids dry, now you have CO2 problems, you need a scrubber in the coffin and you need strong enough walls to deal with the pressure differential, that creates bulk and makes it more difficult to move through the restrictions, and if anything goes wrong the kids suffocate and die and there's no access to them at all.

From the perspective of a trained cave diver the solution that they used was nearly perfect.