Regarding wideband signals muxing/demuxing Yes, modulate then FFT for each signal, do the upconversion and mixing in frequency domain, then a single large IFFT

Regarding wideband signals muxing/demuxing Yes, modulate then FFT for each signal, do the upconversion and mixing in frequency domain, then a single large IFFT to give the final bitstream. Roughly the inverse of this (single large FFT, filter in frequency domain, and IFFT individual signals) is how the 'wideband sdr' at utwente.nl works to support hundreds of simultaneous receivers on a single gpu. The amateur radio folks working with these techniques on HPSDR hardware were calling the technique "Direct Fourier Conversion" but I do not know if they ever got to a releasable state. They spent an awful lot of time prematurely optimizing their stuff to run on the original Jetson board which was not really adequate to the task, so I think that was likely frustrating and killed momentum.