Since the original 555 was developed in 1971, I'm wondering if the inventor[1] or Signetics, the company where he worked, would have made a similar physical mock-up in those days before electronic circuit simulation software existed.
Yes, almost certainly. Designs were routinely breadboarded into the 1980s. Maybe discrete transistors, maybe a mix of transistors and some simpler ICs. Even big designs got the treatment -- the Intel 4004 creators speak of a breadboard prototype in one of the oral histories.
My impression is that such prototypes were something given up quite reluctantly. It's the best way to verify your design works, after all. But eventually ICs developed enough. Either so tiny in process, that they behave electrically very different from circuits with macro components. Or simply so large in design, that it would cost more to build a discrete component prototype than just do a prototype mask run.
Probably, although by then you could get op amps and flip-flops as ICs. A 555 timer is two comparators, a flip-flop, an inverter, and a power transistor. If you have those building blocks, you can make one easily.
[1] Apparently it was a single electronics engineer, https://en.wikipedia.org/wiki/Hans_Camenzind