Huh, strange. Everywhere I looked online indicated that this pair of headphones has a single TRRS 3.5mm jack that comes with a usb adapter.

TRS (tip-ring-sleeve) and TRRS (tip-ring-ring-sleeve) indicates the number of channels on the 3.5mm jack. TRS is usually dual channel balanced (one channel for ground) whereas TRRS includes an additional channel for a mic.

As for all the other cable stuff, everything you've told me means that the troubleshooting for jacks and cables can stop there (I'm also assuming the echo happens in both ears and not just one). If we can't replace the cable, we can't tell if the problem is the cable or the headphones. I doubt the USB adapter will help either, unless JBL has some weird thing going on with that.

The echo being affected by your headphones volume control makes me think the internal wiring is miswired somewhere along the signal chain. You should still be able to use the headphones without the mic, so that's what I'd do.

I'm planning, in the far future, to buy a cheap DAC to plug-in a headphone, a microphone and an electric guitar so I can use all of them on the computer at the same time that I can use the headphones as monitors for the microphone and the guitar. Is this possible?

You're looking for an Audio Interface, not a DAC (although audio interfaces have DACs in them). There's a lot of options out there, and you'll have to assess the options based on your budget and use case.