You can launch Fallout 3 without running into SecuROM. If you use the Fallout3.exe directly, it loads the game straight away. The launcher, I believe, asks for the disc.
In regards to modding; It depends on how the developer has used the engine. Divinity 2 uses Gamebryo, which is used in many moddable games (Oblivion, Fallout 3, Civilization IV, Freedom Force), none of which are reduced in "modability" by DRM measures. If Larian have changed the way the engine functions to disallow mods, then it would require someone of talent to be able to make either a doorway for the mods or a tool in which to make them.
