Sorry guys, didn't have the time yesterday to do screen print-outs, I was on a production deadline.

For the Scanner, I used a Canon scanner, an HP Scanner and the best results (in terms of numbers) I'm getting from the Epson scanner.

I will post histograms and also try to see if I can find saturation and color settings in the initial screens of PP that could correct scanner lighting issues. However, by having tried this before, you sort of have to do this for every paper you profile.... 

Good news then  
Some large format "pro" printers have a driver adjustment for the amount of ink but on many models, the only way is to use different paper settings. I think the rule is: more glossy, less ink and visa versa so matte paper uses the most ink. In your case, semi gloss uses less ink than lustre.
I would say yes to that rather than trying to tweak things before making the profile, especially as you are getting very close now. The edior enables you to see and compare 2 images side by side. One is the "screen" image, as it should like like in print; the other represents the print and is the one that shows your adjustments. The aim is to get the "print" image to look like the test print with its errors. PP then adjusts the profile to correct the errors. It sort of works backwards from normal image editing.
The editor includes colour wheel and you can see which segment of the wheel into which a selected colour falls. I don't think you need worry about which precise grade of yellow it is, just have a go. I found it takes some practice so be prepared to use some ink & paper  
Probably  :-\
You may need to ignore the RR paper type recommendation, and work one step towards glossy as a starting point.
Keep us up to date, others will find your learning curve helpful too  
Terry.
PS. Don't forget to set your driver as per the new paper settings when printing. I'll say it again, use Qimage so you can save every single print set up so you don't have to remember the detail.