As said already, the first is a MPi Abarth, this is in the UK at least a standalone model that had the side skirts, rear spoiler, Abarth stenciled on front bumper mesh, headrests with Abarth badge, 14" wheels with stud conversion and 10mm spacers at front and a different final drive in the gearbox this is important as makes it a mechanically different car from the sporting, it also has red stitching on steering wheel.
There were also optional extras, Abarth sill covers, multi spoke alloy wheels rather than the 8 spoke versions, headlamp covers, alloy pedal covers, alloy gearstick surround, mats, and probably a whole load of other stuff.
The Schumaker is same as above but has Schumi badges rather than Abarth and has no red stitching on steering wheel IIRC. But it also has the alloy pedal covers and alloy gearstick surround as standard.
The earlier SPi models are really just a Sporting with optional extras, normally either specified by dealership to promote model and make it stand out, or by customer on purchase.
It can have any combinations of the following but normally what people call an Abarth have the minimum of side skirts, spoiler, headrests with Abarth, 14" wheels. There is no red stitching on steering wheel, no different gearbox to compensate for bigger wheels, so its slower than Sporting model with 13" wheels, and therefore has no mechanical differance from normal model. Again sill covers, a leather two tone steering wheel with Abarth horn push for non air bag models, pedal covers, mats, headlamp covers, 14" wheels etc etc were all offered.
A friend of mine up in Glasgow bought a Sei a few years back, it has full Abarth body kit, really nice Abarth sill covers I have not seen on any other car, headrests etc, but still had 13" wheels and he had original bill of sale and these items were optional extras all added on purchase.
There are often old brochures on ebay that show all the items that were available either at sale time or afterwards from the Fiat Collection I think it was called, has a pic of Yellow Sei with Abarth kit on front page.