Just posting to let you know that I haven't forgotten about this and I've been trying to find an optimal solution to the black screen issue. Sadly it's not resolved just yet. Sorry about the delay.
Viewing post in Ritter Ultimate Event Spawner (RPG Maker MV) comments
Hello I've uploaded a version 1.3 of the event spawner. This update fixes the black screen bug when adding a bunch of pre-load maps to the plugin parameters.
As far as the other issue with the last spawn eventId... that's still unresolved as of now. I've poured some time into trying to find a solution to this specific problem you're having but it's more specific to the way you're trying to access it rather than the command itself failing. I'll continue to try to find a solution for your needs but it may just be the method of its use in this case causing the issue.