They solved the comms problem on Mars with relay satellites. Would those be harder to use, around the moon? "Habitat space" is useless unless you can get there, and if you can get there, you can get back to Earth.
To disprove it, you would need to find something useful for SLS to do.
NASA, of course, has no say in whether to use SLS. Congress decides that. The best NASA can do is have it not ready to fly. The longer it takes to fly, the fewer flights they will end up forced to do.
If anything the Lunar Gateway sounds to me a lot like "classic" Nasa risk aversion and over-preparedness. It isn't strictly necessary, probably, but it sounds like it makes lunar projects overall safer (in communications access at the very least, but also reusing ascent stages, and providing more backup habitat space in cases of disaster readiness) and that's still good science to prepare for risk. (They don't want a second Apollo 13, and who can blame them for that?)
> The only reason to even consider using the high moon orbit is to have something, anything for SLS to do.
It seems like that is already disproven. The first few missions to setup Lunar Gateway modules have already been scheduled on SpaceX Falcon Heavy launches (in 2024) so clearly even Nasa understands they may have to rely on SpaceX even for access to Lunar Gateway orbits because the SLS has already missed deadlines. The "traditional" flipside to political boondoggle politics is "standard" commercial bids and contracts, and Nasa is far too pragmatic to rest important pieces of a scientific mission waiting for the SLS as the "only option".