I think our inclusion of NS5 was just a temporary bandaid to a larger time problem that EU4 seems to suffer from. Each timing category has its own issues, so it's hard to standardize. Regular RTA (with speed 5) is affected a lot by computer speed; I think mine runs it at ~3sec a month while others can get to 2 (which might be the hard limit for speed 5, though we don't know for sure). Compounding over the course of a hundred years, that adds an extra 20 minutes, not including other times like autosaves. For speedruns, that poses a problem since runs can be the exact same but one is still slower than the other due to the user's computer.

RTA with no speed 5 is designed to decrease the impact of computer speed, but it doesn't completely remove it, since a faster computer still has an edge due to the way speed is dependent on your computer in EU4. Also, with long runs like WC, NS5 could end up making it up to 3 times longer, which would theoretically make runs more than 24 hours long (using the current WR to compare). Forcing everyone to run on a speed 2-3 times slower than normal can be rough, especially for the longer runs.

IGT isn't exactly a true speedrun, since it shares more similarities with TAS runs, but I think it has its merits. Paradox games are inherently going to use IGT as a standard, so it make sense to "speedrun" in such a way. RTA is still the standard speedrun category, but I don't think there's much harm in leaving IGT as it is.

Ideally, in the future there would be a program that could remove autosave times and convert all runs to a standard speed (by measuring average time per month and converting every run to a standard baseline monthly speed), but the accuracy of such a method has yet to be tested. For now, we're focusing on filling up the empty categories, since there's no point in solidifying time rules when there's no runs to be ruled on.

Also, have you joined the discord? I think it's more conducive to ruleset discussion than the forums, so if you get a chance, hop on in and share your thoughts.