When I have a complex model that I want to texture, (eg a character model),
in max I will select faces that may need different mapping cooridinates.
I'll set the material id for these faces (more on this later) to a unique
number, apply a uvw map just to those faces, picking a mapping type that
"best fits" the faces (eg for a head I will use a cylider, for a shirt
I might pick the front faces and apply a planar map, and do the same for
the back). Once I have UVW mapped the entire mesh, I apply the
unwrap uvw modifer to the mesh. When I go to the edit window of this
modifier, I can use the material ids I set when I was first applying the
uvw maps to "isoloate" those faces and move them aside, once
I have moved all of them, I can go back scale them and move them exactly
where I would like them to go. This way, I only need 1 texture map for the
entire mesh, even though that mesh may be comprised of a number of elements.
You may already know all of this so sorry if I'm telling you something that
you already know.
I think the key here is that this allows me to use 1 material and one
texture map from within 3d max to texture the model.
It sounds to me like you are applying a lot of different materials with
diffuse maps to the mesh.
I'm not sure about having a model with multiple textures in 3DGS.
We use torque for military sims where I work (I like GS much better

)
and I am able to export a single mesh with mutliple materials/diffuse maps.
I'm pretty sure that in Unreal it is common to use one texture map
for the body and one for the face, but I think you can only use 2 total.
Regardless, I always try to use one texture map. Using unwrap uvw makes
it possible to texture an entire character model with one image.
If you would like, you could zip the max file and all of the
textures you are using and post it somewhere that I could download.
I can take a look at them and see if I can get a better idea as to
what's going on. (I have max 8).