It is quite hard to tell from the image that you have posted, but to me it looks like a result of the printing process being unable to reproduce deep or bright colours on the media being used. This is very common on matt and uncoated media.
I would expect LAMINATING (or wet varnishing) to help the colours to "pop"
You should be able to do a test of this by ...
The trap (or grip as some people like to call it) should be a continuation of the shape that you are trapping, so the overlapping area should always be the same colour and tint as the rest of the shape.
To put to another way; If you were to take away the object that your shape is trapping to, then it should just look like a slightly larger / expanded ...
What you see on your monitor is RGB. Yes, it's a CMYK file, and it's likely trying to compensate, but it's still using an RGB model.
As such, what you see on screen is usually always more vibrant/saturated than what CMYK can produce. This is due to the reproducible colors being different in each color space:
Using coated paper and/or applying a top-coat ...
and welcome to the GD StackExchange site.
Package design is always an interesting, challenging task. Let's break down your requirements into two separate answers:
1. How do I know how many inches the bag should be?
Generally this should be provided by someone to you. Most likely the person/company that will be manufacturing the product would provide this ...
Couldn't you just set your black to grey scale Black K and then adjust the black to the grey color you would like. Just a thought.
You currently have your pantone as black U 85%
change it to grey scale
Side by side comparison:
Now your using the color black at 70%
Maybe I'm not understanding 100% what you are trying to achieve.