Hi Bamm98,
Welcome to the Mattress Forum! 
I’ll do my best to throw a few things out there for you and see if any of it makes sense. Unfortunately, attempting to predict what arrangements of layers might work best for you via an online forum is quite difficult, because of the very individual and subjective nature of each person’s sleeping surface preferences.
First off, I’d make sure that you reevaluate your pillow to make sure that it is providing a decent alignment to keep your cervical/upper thoracic region in a relatively neutral arrangement. Improper pillow thickness is a common cause for shoulder issues, especially with a new mattress.
Next, you may wish to reevaluate how you are sleeping upon your side and the location of your arms, as this can also make a large difference in shoulder pain, regardless of mattress types. While on your side, fold your arms across your chest (like what is commonly depicted on an Egyptian sarcophagus). So if sleeping upon your right side, your right arm would be crossed over your chest, with the elbow pointing down toward your waist area and your hand closer to your left shoulder. Doing so adbucts your shoulder girdle and takes some of the pressure off of your shoulder joint. Also, consider placing a thicker pillow in front of you and place your free arm (in his case, the left arm) a top this pillow to help support it in a more neutral position and take pressure off of both shoulders.
Now, for the latex layers.
The first thing I would have you experiment with might be counterintuitive, but it could provide you some relief. I would have you try switching the order of your current layers, and place the 28 ILD layer on top of the 19 ILD layer. Sometimes people can use layers that are “too soft” on top, and they in effect “feel through” them to the deeper layers, which in this case would be the firmer combi-zone innerspring unit, and if this would be the case in your particular situation then you’d feel more comfortable with this “dominant layer” construction, and adding even softer layers would be counterproductive. It’s a bit of a long shot, but as you have the layers at home already, it’s worth a shot to try and it doesn’t cost anything.
As for the difference between 28 and 24 ILD, yes it is generally noticeable by most people. Using the 24 ILD in the place of the 28 in the second layer of the product would result in a “slight” difference in overall plushness and sinking in.
I wasn’t a part of your phone conversation with Ken at Arizona Premium (I’m happy that you’ve had great dealings with him – as you know, I think highly of them as members here on the forum), but I certainly would put stock in his recommendations, as he is quite knowledgeable about his products and latex layerings in general. Apparently, from what you two discussed, his thought was that based upon your experience that swapping the 28 ILD for a 24 ILD wouldn’t be enough of a difference for your needs, and swapping the 28 ILD for a 19 ILD would be more appropriate.
Going with a 14 ILD and 19 ILD configuration would be quite plush (regardless of ordering), and while that certainly would create a very plush surface comfort, it could cause upper alignment issues and also create a more “abrupt” transition from the very plush comfort layers to the innerspring unit. There is more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel”.
I understand your desire for the ability for multiple layering options, but personally my focus would be mostly on getting the comfort correct. I would have a concern of the 15/19 combination being a bit too plush for your mass, and with the options you’ve presented I’d personally be more “comfortable” with the 19/19 or the 19/24 recommendations.
That’s about the best advice I can offer via the forum. I hope that information is helpful to you.
Phoenix