There are a variety of issues with your model that are likely causing your frustration. Mainly they indicate that you need to try to shut off your CAD instincts and study SketchUp as its own beast.
The most serious issue is that there are multiple examples in the model suggesting you are not carefully using SketchUp’s system of inference snaps and locks. There are edges that are offset by a tiny amount from adjacent ones, as if you didn’t wait for an inference snap tooltip before drawing or aligning them. There is at least one interior wall surface that is slightly askew from the axis and not parallel to the outer surface, as if you didn’t lock the direction when you drew it. I’d suggest that you practice drawing while paying very careful attention to inference engine tooltips, snaps, and locks until they become second nature. The tiny errors that result from not doing so often lead to mysterious glitches that are hard to find and to correct. Also, while you practice this I’d set the model units to as small as available so that tiny discrepancies are revealed when you tape-measure something or add a dimension.
As @eric-s noted, you are using SketchUp’s layers as if they are 2D CAD layers, which they are not. SketchUp’s (unfortunately named) “Layers” are tags that can cause multiple objects to be displayed or not with a single setting, but they do not in any other sense gather, isolate, or structure the objects. You can associate edges with two different SketchUp layers and they will still “stick to” and intersect each other and the faces they bound.
So, drawing some edges with the “Existing Structure” layer active and others with “Outline” active does not isolate them. You use groups and components to isolate collections of edges and faces that constitute a single “object” from others. Once you create a component or group, you can assign a layer to it to affect its visibility, but it isn’t necessary to assign the same layer to its contents and doing so can actually cause confusing issues with visibility. All edges and faces should always be drawn with Layer0 active and left that way. And there are only special circumstances in which you should make anything other than Layer0 “Active”, none of which apply to models such as the example you attached.
Regarding solids, in SketchUp a “solid” is a group or component that contains a minimal collection of faces that completely enclose a single volume of model space. By minimal, I mean that there are no extras such as “flaps” or internal partitions and also no non-face edges. Your outside walls component has some examples of both extra partitions and stray edges. Many of these look like they resulted from the small offset edges I mentioned above. By completely enclose, I mean that there are no holes connecting the interior to the exterior (called “surface boundaries” in some discussions). In you sample model, the walls do not have tops so they fail this last point. There is also a problem with a portion of the bottom of the walls due to one of the slightly offset edges not meeting cleanly at a corner.