While the engine can handle models with that poly count, it would be much better to work with a model designed to be used in games/realtime. The poly count just isn’t necessary and you’re going to have issues working with it as you can see.
Still, as far as UV mapping goes, you can unwrap it, but you have to do it the right way, no automatic method will give you as good of results as doing it by hand. You may need to do some general UV unwrapping tutorials.