Any KZ Durango Gold owners? If so, a couple questions

Looks like they should both pull out. https://youtu.be/Y20KsQapkW8
I have a 2012 kz TT. I got it used. I am impressed with the build quality for a TT compared to the other stuff i looked at, that could have changed by the time yours was made. Check the couches and make sure something isn't broken. Or that your doing it corectly.

It sounds like you simply need to pull the sofas and mount them correctly. These things are just a heap of cheap materials that need assembly.

From what I have been able to find, Durango is about the only maker that uses actual plywood on their floor vs everybody else using OSB.

When buying a used unit, the condition of the particular unit is far more important than the initial build quality. A 5 year old well built unit in poor condition is unquestionably a worse buy than a cheaply built 5 year old unit in pristine condition. Water intrusion is the RV's worst enemy and some water damage is very expensive to repair to the point costing more than purchasing a new unit. You mentioned the slide having a 4" gap. Not sure where and if extended or closed, but that at the very least shows the slide needs an adjustment. It also could be water intrusion has destroyed some of the bracing causing the misalignment which is common with slide outs.

“we noticed today that when we tried to pull out both sleeper sofas, there was not enough room!!. Only one sofa could be fully pulled out.”

lol. That a good indicator of design quality.