I've spoken with the Dev for the mod; DF needs to be installed either in the /Mods folder with the 7 Days installation, or it can be installed in the UserDataFolder. Both should work. It works fine with single-player, and it works fine when running a LAN game, so it's definitely on the SS side of things, not the mod's. As we as users cannot access the UserDataFolder, our only option is /Mods within the installation.
That said, when I requested the mod be updated onto my server, I basically got shot down. Told unequivocally that "it will not work." The mod developer is aware that people are having issues here with SS, but also that other hosting sites are fully able to run and support the mod. This is not an issue with the mod. It's an issue with Survival Servers. I've now been paying for a server for two months that I cannot play as I wish to play.
To be frank, Darkness Falls IS a resource-heavy mod. It takes up larger amounts of RAM, I get it. But honestly, that feels like what's going on here. I've been a Survival Servers customer for YEARS. I remember when there was a price increase based on the size of the world one was running. No notice, just a sudden inability to play on a world you'd already had built if it was "too large" for your monthly subscription rate. To be fair, SS made that right in my case; that's why I've continued to be a happy customer.
Now it feels like we're getting a bit gaslit that DF "doesn't work" when in reality, it seems as if it might be a resource/cost issue, especially as I'm not being helped to add the mod if I'm willing to accept the risk that I might "fall through the world" or suffer other adverse affects. I'm just being denied. I'd be fine with paying a bit extra for DF-based servers, if that's the case...but this "the mod is just broken" line feels really disingenuous. It may NOT be, but the way it's been presented puts ALL the onus on the mod itself...and that's just not where the issue seems to lie.
EDITED TO ADD: Customer Service has agreed to add the mod; so we shall see how things progress. Will update as warranted.