[robespierre]

Is the strictness of the o32 ABI a reason why there's two linkers for MIPSPro? One handles both 'new' ABIs, the other is dedicated to o32.

I don't know at that level of detail; this may be a question only the authors can answer.
The new ABIs were needed to support the added features of the R4000 and later processors, like 64 bit GPRs (even in 32 bit address space for N32), 32 double-precision FPRs instead of 16, and multiprocessing stuff.

A weird aspect of MIPSPro from my relatively amateur experience is that you're limited to using the cc driver with o32 and thus c89. I suspect that MIPSPro as we know it is partially composed of IDO-era holdovers for o32 and was only rewritten/reworked for n32/n64 ABIs. I could very well be incorrect here however so I'm not stating this as fact but only a theory.

Again, I don't know, but it might have required more work to support some c99 features like variable length arrays on the older ABI.