Well, if the mod dumps as soon as you just replace the mesh entry, then its most likely what I call the Freaking Spelling Bug. They are HARD to spot. I thought there was something wrong with my eyes, because it took a dev checking out my files to spot that I had one letter of the wrong size.
Also, check out the mesh file. If you make any misspellings there, including case sensitive errors, it will also dump on load. I ran into that little issue when I was using the early version of the planet resize tool and it wrote one of the entries in the .mesh file with incorrect capitalization.
I doubt that it has anything to do with the model itself. Early on I had all sorts of messy models imported into Sins and it didn't crash on me once, it just goes all wonky with the graphics of the ship.
Basically, you have a very narrow area to search for the bug, if the only change you did was to replace the mesh reference in the entity file. If you haven't already, check that out by returning the mesh reference to the vanilla and see if the game crashes. If not, you've got your culprit.