I figure that Bethesda will probably release the GECK just a month or two after the initial release of the game, like they have on Fallout NV, and the Creation Kit for Skyrim.
The engine, knowing Bethesda's glitchy engines (understandably, as their games are huge open worlds) may have a some bugs in it, but they say that the game itself is done development wise, for now, and they are just fixing bugs until it's release.
As for the Tamriel Online mod, that looks great, and I do hope that the author, or at least someone, when the mod is further into it's development, ports it over to Fallout 4.