If you attempt to play back a demo in Doom that has >50 movement it will say that the "Player is Turbo". Also the LMPC is an already existing tool that provides stats on an LMP file, including how many tics are >50. In general verifying an LMP file falls within certain restrictions is pretty trivial.
Using JPC-RR might create a run that is 'acceptable' to this site, but it is adding overhead and difficulty of making an LMP run that existing tools (such as the Doom Replay Editor) do not have, which would probably result in a lower quality run.
And I also agree that there shouldn't be an assumption of performing a run in nightmare difficulty. Certainly a fully TASed/Built nightmare run would be awesome to watch, however nightmare and ultra violence are two very different games, which is why the Doom speed running community has always seperated between them. Although due to the lack of built nightmare runs the bar is a lot lower for creating a 'record' time, so that could be one reason to choose nightmare.
I lurk around on here and the SDA a bit :), just don't post much
Calling the current records not optimal is true, but improving upon these records is a tall order. The current fastest TAS run is 30uv1402, which you can also find on YouTube. This run can be improven as shown by this run, which is the first 13 levels done approx. 30 seconds faster than 30uv1402.
Not saying these runs can't be improved, certainly they can. That half run is (as far as I know) optimal up to the point it finishes, although there are some tricks in later levels that could be implemented which would make it perhaps up to a minute faster than 30uv1402. However the effort required to replicate and then beat these runs would be extremely high.
Of course, I would love to see something faster than these. So if you want to go ahead with this then I wish you the best of luck, just as long as you know where the bar is.