I’m a newbie so don’t take my answer as the right one, I haven’t use SolidInspector or make a project like that yet.
You maybe can divide some of those groups into components and work with them, I think it’s good to get used to work with components but wait for others answer.
I’m going take a look to the .skp, if I find something I will tell you.
With solid inspector you can use the Tab key to zoom in on each error in turn.
So here I have used Tab within the group edit mode to find the problem then select and delete it.
Not sure if this can help, but for getting area I think you need a closed form.
I don’t know why it tells me the area for individual components but it doesn’t tell it when I select more than one.
You can explode your components, make new ones or make components with other components inside.
Tools > Outer shell could help you to remove the inner walls.
I repeat I’m a newbie, so I expect not to address you in the wrong direction.
The 4 entities causing the problem are a tiny triangular face [grass-material] and its three sides.
One side is less than half a mm long - only 0.000395" long, so you can’t even see it, or it highlighted etc - SketchUp tolerance is 0.001" - 1/1000" !
How this tiny sliver got left floating near the main geometry is a mystery…
Here it is having selected it and used Edit > Zoom Selection…