IRS suspension

[GS guy]

G - seems like you're confusing suspension pivot points with drive axles.
A swingaxle drive axle only has 1 pivot point - at the differential. The suspension pivots use that as one of the pivot points, and the 2nd at the torsion bar - as you described.
While the swingaxle and IRS designs share similar numbers of suspension pivot points (at 2 as you stated), the IRS semi-trailing arm has the advantage of Significantly better camber control through the full suspension travel - at the expense of added weight and complexity. It also provides a controlled amount of camber gain in roll, along with a lower roll center. The reason it can't be used in a car like the Spyder is you'd have to locate the inner forward pivot point inside the engine! You could, however, adapt a 914 trailing arm design and use the IRS style trans - again at the expense of added weight and complexity.

There's a reason all auto manufacturers got away from using the swingaxle suspension (with some rare exceptions - like the Ford twin I-beam pickup front suspension) - roll center is too high and too difficult to control the huge camber changes while maintaining decent suspension travel. It certainly is less expensive and much simpler to build from a manufacturers cost perspective!
However, in certain specific applications (smooth pavement and very limited suspension travel) it can be made to work well - especially with added features like using a Z bar or ZRS design.
Jeff