Thanks Christophe, glad you enjoyed the tutorial.
Scanner profiles don't assure accuracy of colour rendition for colour negatives because the film can't really be profiled with icc profiles in the same way positive transparency film/scanner combinations can be. That is why they developed Negafix. The fact is, that you need to use a separate Negafix profile for each colour negative film type you are scanning. And this goes not only for the type, but also for the ISO sensitivity within each type, insofar as - for example - a Kodak Gold 200 will not render colour the same way as a Kodak Gold 400. If you are using SF Ai Studio, there is an Expert Mode which allows you a great deal of customization potential to fine-tune the Negafix profile very correctly.
The most workable combination of settings in CMS I've found is to use your scanner profile as the Input profile, and use Adobe RGB(98) for the Colour Working Space for the reason you mention. OR, you can scan with no profile embedded by chosing one of the HDR modes (and going into CMS and putting <none> everywhere, in General Preferences making) sure your Gamma is 2.2 and check the box "for HDR output" if you are making an HDR scan). Scan with Negafix working, then when you open the image in Photoshop you will get the missing profile warning. At this point I would Assign the scanner profile and Convert to the Working Space - selecting ARG(98) in this instance.