Michelle,
>>union, mirror, attach, and seams…
I'm a little late on this (first free 1/2 hour in weeks!), but if I understand
your problem, there is something else you can try. Assuming you couldn't get
the boolean to work, and have done the mirror and attached the two halves, all
you may need for a seamless boundary is to weld the vertices.
If you have mirrored symmetrically, the vertices should be aligned exactly if
the two halves are positioned correctly (zoomed way in). Just select all the
mating vertices and Modify/Vertex/Weld "Selected", perhaps turning the Weld
Threshold up some first. Alternatively, the vertices could be welded manually
since it looks like you don't have very many.
Once welded, smoothing can be applied across the boundary and there will not be
a seam.
BTW, when AutoSmoothing: I usually do a Modify/Edge/AutoEdge (with AllLines
turned on) before auto-smoothing to "preview" the smoothing. If a vertex isn't
properly welded or an angle between two faces too sharp for the desired
auto-smooth, Auto-Edge will show the offending boundary as a solid line.
J K J