Anyway, it seems to me that the thread has gone way of course. The OP made no mention of faded slides. He did ask some specific questions though, and I am not sure if we have answered them all?
The OP mentioned digging out 15+ year old Kodachromes and Ecatchromes. They'd be faded to some degree. As I recall, Kodak Q60 targets should not be used if they are more than 6 months old. I actually have a Q60E3 target. Whilst searching for some slide material which is not significantly faded so I could test David Good's suggestion of scanning into Device RGB instead of the usual color spaces like sRGB or ProPhoto, to get a clearer idea of just what the differences are, I came across it, neatly folded in an envelope. The date, 1999:12.
I didn't see much point in using a 6 1/2 year old target as a reference point for calibration, so just used it as a normal slide to scan at differnent settings. The results seem quite normal to me. No color cast that I can notice. Natural skin tones on the lady on the right. All the colors and hues are clearly delineated, including the brightest and the darkest, the most saturated and least saturated. And, of course, there's plenty of opportunity for grey balancing on the Q60 target, which was another indication of the lack of a color cast because grey balancing wasn't required.
Scanning into Device RGB results in an image without embedded profile so the appearance is dependent on what profile is assigned when opening in Photoshop. Assigning ARGB seems to produce the most natural result. Assigning sRGB is too desaturated and ProPhoto RGB too saturated.
I get no sense that my scanner needs calibrating. I haven't got time to post the images right now because I'm due somewhere else shortly.
Of course, now I've found my Q60 target, you can be sure I'd at least make an attempt to use it to create an IT8 profile, just to see the magnitude of the difference it makes with the 5400 II, if for no other reason.
However, I failed to successfully implement the calibration. The Vuescan instructions on this process seem unclear. I'm supposed to rename a data file to scanner.it8 (which I downloaded from Kodak) and copy it to the VueScan.ini folder.
Problem is, my computer can't find any such file. I did a search for files and folders. There's a Vuscan folder of course with a heap of files of various descriptions, but no VueScan.ini. This is something I'd have to contact Ed Hamrick about, but I'm reluctant to waste everyone's time for the sake of being able to profile my scanner with a 6 1/2 year old target, especially considering I've seen no evidence that such calibration would help me get better results with faded Kodachromes and other faded film types.
Nevertheless, the Vuescan software does contain 3 slide profiles, Kodachrome, Ektachrome and Generic. None of these appear to be activated. It makes no difference which I choose, if any or none. If I'm able to use these 3 profiles as a result of an IT8 calibration, then that clearly increases my options and it could be worthwhile buying a recent Q60 target.
A google search on the subject of calibrating with an IT8 target, specifically using Vuescan, gives me the impression there's a lot of confusion here. I'm not the only one who finds Vuescan's instructions here inadequate.
I should add, I have no problem scanning negative film, except sometimes when I can't find the profile for my film type. If the software includes a profile for the specific film type I'm scanning, results are close to perfect. If it doesn't, results can be miles out.