Right. My experiment used 1 GB for Stockfish, which would also work on a 486 machine (although at the time, it was almost unheard of...)
(a) The most recent data points are from CCRL. They use an i7-4770k and the listed tournament conditions. With this setup, SF11 has about 3500 ELOs. That's what I used as the baseline to calibrate my own machine (an i7-7700k).
(b) I used the SF8 default which is 1 GB.
(c) Yes. However, the hardware details (RAM, memory bandwidth) are not all that important. You can use these SF9 benchmarks on various CPUs. For example, the AMD Ryzen 1800 is listed with 304,510 MIPS and gets 14,377,000 nodes/sec on Stockfish (i.e., 19.9 nodes per MIPS). The oldest CPU in the list, the Pentium-150 has 282 MIPS and reaches 5,626 nodes/sec (i.e., 47.2 nodes per MIPS). That's about a factor of two difference, due to memory and related advantages. As we're getting that much every 18 months due to Moore's law, it's a small (but relevant) detail, and decreases the hardware overhang slightly. Thanks for bringing that up!
Giving Stockfish more memory also helps, but not a lot. Also, you can't give 128 GB of RAM to a 486 CPU. The 1 GB is probably already stretching it. Another small detail which reduces the overhang by likely less than one year.
There are a few more subtle details like endgame databases. Back then, these were small, constrained by disk space limitations. Today, we have 7-stone endgame databases through the cloud (they weigh in at 140 TB). That seems to be worth about 50 ELO.
Regarding (1): Of course a step is possible; you never know. But for arithmetic, it is not a step. That may appear so from their poor Figure, but the data indicates otherwise.
True. Do these tests scale out to super-human performance or are they capped at 100%?
Except if you have an idea to monetarize one of these sub-tasks? An invest of order 10m USD in compute is not very large if you can create a Pokemon Comedy TV channel out of it, or something like that.