except for the fact the the textures were all flipped on the x-axis.
I think what's actually happening is, that the model's geometry is flipped on the Z axis. The usual transformation setup used with OpenGL uses a right handed coordinate system. If the object has been designed saved for left handed, one axis is swapped and this also affects the texture application of course.
Things like replacing a coordinate with 1 - coordinate don't really work
Yes it would, because…
since that would flip the whole bitmap, not just the current triangle.
… that's exactly what you'd require, if doing it this way. But first I'd check if the model geometry is loaded correctly, i.e. with the right handedness. If not, I'd simply flip the Z axis of the geometry.