Is manually comping actually going to be easier (let alone, give better results) than inpainting? I can imagine it working in simple cases, but for anything involving 3D geometry you'll likely run into issues of things not quite lining up between the first and second image.

100%. Multimodal images surpass ComfyUI and inpainting (for now). It's a step function improvement in image generation.

I'm hoping we see an open weights or open source model with these capabilities soon, because good tools need open models.

As has happened in the past, once an open implementation of DallE or whatever comes out, the open source community pushes the capabilities much further by writing lots of training, extensions, and pipelines. The results look significantly better than closed SaaS models.