The reason this is giving you so much trouble is that the very approach from square one is flawed. You’re trying to make the entire seat interior or exterior from one single surface, and just from a philosophical standpoint, the problems you’re having a direct result of that. Notice how your isocurves pinch in at the sides of the chair? This is called a singularity - meaning the points all end up stacked on top of each other there. There are a FEW instances where a singularity is fine, but this isn’t one of them. You’re essentially trying to describe a circle (the smooth perimeter of the seat) with a rectangle (NURBS surfaces should ideally be 4 sided).
There has been some confusion/discussion in the past on here as to when you should use trimmed surfaces, and when you should use untrimmed surfaces. This is a perfect example of an untrimmed surface being the wrong approach. If you look at the attached screenshot, you’ll see how I would approach this from a “patch layout” standpoint. Really, when you get down to it, the very first thing you should do for every model is try to imagine how your patches will be laid out, and where if needed you’ll have trims.
You can see that the bottom of the seat is made of one surface, that is trimmed by a second surface that forms the back and sides. Then, I’ve trimmed the upper corners of the sides so that the entire thing is smooth all the way around. Now, I’ve done this without any regard for the finer details of the shape you’re looking for, but this illustrates the correct approach to this problem. This surface will offset very easily, and will blend between the inner and outer surfaces with no problems.
Another point I would make is that for smooth/organic shapes like this, your best friend in the entire world is point editing. By using NetworkSrf to create your base surfaces, you’re really locking yourself into a terrible workflow, where it’s just curves in -> very dense/uneditable surfaces out. That’s probably a bit more of an advanced topic to tackle here, but the way that NetworkSrf has become a default/go to command for so many is baffling to me, and I wish someone had told me far earlier “don’t use NetworkSrf as a go-to.”