It's tough if you can't open the model. Is Revit by any chance able to show you any of the errors? (I know it's a long shot, I usually get the "no view available" response.) Usually the unjoin errors aren't critical. I'm surprised it's preventing the file from opening.
Does it open in an IFC viewer? If you haven't tried yet I find both Solibri Model Viewer and Tekla BIMsight to be quite good for this.
Edit: If all else fails you can break up the model by exporting it in pieces to see which ones give you problems. It would probably be easiest to do this by layers in a binary search. Export half the layers. If the problem exists export half of that. Otherwise export half of the other layers. Continue by halves until the problem layers are isolated. You can then continue the process with the elements on those layers. If it's a big model this could get a bit tedious. Make notes as you go so you remember how the layers are split up and which are creating problems.
