Maybe this can help with the geometry:

typedef struct NAME{char   name[NAME_MAXSIZE];} NAME;
typedef struct VERTEX{float x,y,z,nx,ny,nz;int flag,flag1;} VERTEX;    // nx,ny,nz = normals  flag,flag1 = for any use.
typedef struct UVVERTEX{float u,v;int flag,flag1;} UVVERTEX;
typedef struct FACE{int material,group,nindex,index[4],uvindex[4],flag;} FACE;
typedef struct MATERIAL{NAME name;... other properties ....;}MATERIAL;
typedef struct GEOMETRY{int nvertices,nuv_vertices,nfaces,ngroups,nmaterials;
                                               VERTEX *vertex; UVVERT *tvert;FACE *face;MATERIAL *mat;
                                              NAME group_name[MAX_GROUPS];} GEOMETRY;

Now allocate memory for the structures (you will need to relocate memeory during the process)

Scan all the posed geometry and fill all the data of the structures, if a face has more than four sides split into traingles and update nfaces.

Once filled all the structures, sort  face[n] by group and then by material.

Try to find seams within the same material then create extra vertices at the end of vertex[] array.
Duplicate the normals for this extra vertice and update the values of face[n].

Now geometry structure has all the data for export.

Stupidity also evolves!