@baz said:

Tig's Split Tools/SplitDonut should do it for you.

Yeah, it's a big help, this plugin. Thanks!

EDIT: Forgot to mention, it doesn't work for circles (as specified, but I believe it would be a nice feature to include circles too). Luckily there's other plugin I'm using for triangulating circles - FaceSplit

Have you checked your FBX export options ?
I think they should look like this ?
Capture.PNG
Otherwise the doubled up faces could double up vertices etc ?

This is only true if you pick a corner of the bounding box. It defaults to uniform.
Picking any other handle scales against its opposite point, unless you hold shift.

My 'Latticeizer' uses all faced-Edges in the preselected Group as the center-lines for its extrusions -
including the perimeter edges, so offset the main face inwards by half the extruded width before sub-dividing and erase the outer edges to move the assumed perimeter inwards, and then the over all size will be maintained when it's a lattice...
In its dialog you can choose different dimensions, materials and whether or not there are 'panes' to the lattice etc...