3) and yes - "throw as much as you can on the black plate so you can tolerate more CMY variation" SIDs naturally vary during press runs (cue Erik) - generally less on long runs but there is still a periodic variation. Here's what it looks like on a 6 color press:
Gordon, did I hear my name.
Variation. What some see as natural, others see as unnatural, maybe even supernatural. Spooky.
Seriously, there are different types of density variation in printing and none are natural. The amount of variation is a result of the design of the press and other issues, but mainly the design of the press.
Variation types of drift, periodic and random are seen all the time.
Drift of density is basically only related to the ink feed rate variation. It can be improved. There is no reason why the average density during a run should not be in the range of +/- 0.02 pts. In fact you probably need this level of control to maintain visual consistency.
Periodic variation is due to a form roller inking the plate and the area where the ink was remove on the form roller comes around again without being fully re-inked and prints a mechanical ghost onto the plate in a different location. It is periodic because it is related to the circumference of the form roller. This to can be greatly reduced or eliminated by the proper design of the roller train.
Random variation can occur because the ghost in the above example goes up onto the roller inking the form roller and comes back down in different locations via different paths. This too can be reduced by proper roller train design.
Your plots I hope, are just to show the periodic variation but the scale of variation is huge. Periodic and random variation will always be present but the issue is its magnitude. That is a press design issue and the level of variation can be designed into the press. One does not need zero variation but just a level that is not perceptible.
The subject of GCR is of interest and I have some thoughts about it. I am not an expert on this at all but I have suspicions that excessive use of GCR is problematic.
First let me say that I have some unproven views on colour spaces. It seems to be assumed that just because one has a mapped colour space, describing a volume, that one can obtain all the colours in that space. I think this is not true and is especially not true with print. There is the issue of discrete screen values and for monitors the discrete levels from 0 to 255 for each channel. These will cause a limitation but I am not talking about that issue.
It is easy to think of a monitor, which has independent RGB channels, which can independently be increased, being able to have much control of obtaining colours within its space.
Printing of ink is different even though one can think of printing with CMYK as RGB. M plus Y give R and so on. One can think of each spectral curve of CMYK as different amounts of RGB. What this means is that with this physical filtering of light all across the spectrum with the CMYK filters, one can not adjust the R or G or B channels independently. Therefore, one can not probably obtain all colours in the space due to this lack of independence.
Getting back to the GCR subject. The idea, as I understand it, is to change a screen of CMY to cmyK or to cmK taking out all of Y for example and have the final colour result be the same as the first. It is a good idea to help reduce variation on press, especially in an industry that refuses to address the fundamental cause of variation.
So what happens here. We start with higher percentages of CMY screens and then go to lower percentages of cmy screens and add K. The trick will be to pick the right combination of cmy screens that will give the desired hue and have K give the gray.
This is tricky since for a single ink, the hue of a print of a screen will change as the percentage of screen changes. Just making a simple calculation of reducing all the screen values by some amount and replacing it with black can have an affect on hue and L value (Lab). So probably colour management is required to find the right combination of cmyK that will get one the desired result.
But maybe the cmy colour is not obtainable for the reasons I gave above.
There is also the problem of the K screen. It now becomes less controllable as it goes from small percentage screen values to higher percentage screen values say in the mid range. Also as the dots of the K screen get bigger, they might interfere with the cmy dots covering some and not others which could alter the hue locally.
Probably moderation of GCR is best and also with the effort to gain control of the types of variations mentioned. One must try to understand that if a press has such variations, it can not have an accurate profile or fingerprint or characteristic data set. One is fooling oneself if one ignores this fact.
If one wants to save ink, one way is to run all ink densities very tightly at the lower control limit. Also by saving paper, wasted due to poor density control, you also save ink. Save paper first.
So many issues in printing lead to the same critical need. That is to gain tight control on density.
