aarghh - for gods sake find and USE the return key !

Okay I will wade though the mass of text....

It's a cheap i3 - pretty much an anet a8 clone.

Have you tried a system restore on your computer to a point just before you initially installed the printer ?
That would be my first move. Even macs can do a system restore - yes really, crap as they are :-)
Be careful if you have windows 10 as it prefers to wipe everything, rather than a simple system restore.
As I refuse to have a windows 10 computer in my workshop - unless it's someone elses ;-) - can't, off the top of my head tell you the system restore procedure.
A search should find lots of videos and articles on it though.

second - sketch up is not great for producing the solid models 3d printing needs.

have a look at openscad: www.openscad.org
Have a look round for some basic tutorials - much easier to use than you originally think :-)

Sounds like either the slicer was adding a solid diaphragm at cwertain levels, or sketchup just bogged the stl up.