I respectfully disagree with the "manufacturer must only make supercars" and "cannot resemble any other model" requirements. Those requirements exclude all Porsches, Maseratis, MBs, Audis, Jaguars, Acuras, BMWs, and Fords, including the 918 (quintessential supercar), Carrera GT, 959, MC12, MC20, GT40, FGT, CLK GTR, MBOne, SLR, NSX, XJ220, M1, R8, GT2RS, GT3RS, GT2, GT3, 992TTs -- just to name a few. IMHO, the above-listed cars exhibit the attributes of a supercar (adjusted for time). Some of these are landmark, legendary supercars. One cannot deny supercar status to the 918, MC12, FGT, CLK GTR, NSX, XJ220, M1, or R8 simply because the manufacturer also produces non-supercars.
The ZR1 (and forthcoming Zora) are tricky. The base Corvette is so ubiquitous and attainable that I too balk at the notion that its big brother is a supercar. On the other hand, it is hard to deny supercar status to such a spectacular performing car.