Editing the terrain without deprotecting a map is possibly the most difficult thing you can do.
The terrain is in the war3map.w3e file. The spec is intimidating and I wouldn't want to try to edit that manually:
https://github.com/stijnherfst/HiveWE/w ... 3e-TerrainRemoving the bridges shouldn't be too bad I think (war3map.doo or war3map.j). You could open up the war3map destructible or doodad files in the World Editor and modify the model on them quite easily. Have you tried any other bridge models? Maybe only some of them crash.
You could try deprotecting it and then creating a shell map with only the terrain changed, then re-import the other files into that map.
Or do the opposite and extract the .w3e and add it to a blank shell map of your own, change terrain, export it, and then reload it into the main game