Yes and no. Even a single-shell data will normally have a few b=0 volumes, and these can be used as an additional ‘shell’ to perform a 2-tissue decomposition. While this is far from perfect, you can use this with mtnormalise, and the results seem pretty good, as far as we can tell. I’d expect the intensity normalisation to be more appropriate this way.
Nope, I’m afraid not. The need for intensity normalisation comes from the fact that different datasets will be scaled differently by the scanner – even for the same subject on the same scanner – depending on the scanner’s calibrations and other factors. It’s this arbitrary scaling of individual datasets that we need to account for, and by its very nature, that scaling needs to be estimated for each subject independently. It can’t be estimated from one (group of) subject(s) and applied to another (group of) subject(s).