To summarize the issue, for the sake of others browsing this thread: unfortunately, the way TheNamelessOne implemented the Movement Multipliers feature, checking that box
and doing nothing else will change the game's behavior. In other words, not only does this
enable additional COSMIC2 keys, but it actually
requires that they be added in order to maintain normal/standard behavior. I wish he had handled this differently, because this has come up as an issue or source of confusion on multiple occasions in the last year, but ah well.
Therefore, if you are playing a scenario that was made prior to TOTPP, do
not check the Movement Multipliers box.
@YazzyT -- your question was about more recent scenarios, though (made since TOTPP was released). If the designer didn't include any instructions about this in a Readme or Notes file, the best suggestion I have is to open up the Rules.txt file, look for the heading labeled @ COSMIC2 (without the space), and look at the lines in that section. If any one or more of the following four labels appears, then
yes you should definitely check the Movement Multipliers box:
RoadMultiplier
RailroadMultiplier
RiverMultiplier
AlpineMultiplier
If none of them appear, then I think leaving it
unchecked would be correct. My reasoning is that if the designer had intended for the box to be checked and override familiar base behavior, it's extremely likely that they would have added one or more of these entries to COSMIC2 in order to fine-tune the behavior (or make their intentions explicitly clear).
It looks to me like Rules.txt for Techumseh's new Burma scenario does contain two of those four values, so checking the Movement Multipliers box for this scenario would be correct.