Is anyone experiencing an issue like this - ive got no where with Adobe Tech support. Library and Development modules display wildly different variations of an image - I get the image right in Develop and then when i export, i get the Version as it looks in Library Mode. Ie much darker. See video and screenshot below.
Im a wedding photographer - edit 1000's of photos, export and randomly find these dark images that are way off the edit version....
I'd suggest you are actually finding a couple of problems albeit with similar renderings. The following is assuming you are using a calibrated monitor, you are on LrC 10.3 and all drivers are up to date.
LrC displays differently between Library and Develop. This is a continous frustration for LrC users. For details see FAQ | Color in Lightroom. There are a couple of FAQ's related to exporting and printing.
In terms of your export issue of "..randomly find these dark images that are way off the edit version", that is strange if others are exporting correctly using the same export settings. I'm not a MAC user but can only suggest confirming that you are using a viewer that supports colour management. What are the parameters you are exporting with?