In my K2 Item page layout, the image designated on the item tab shows as the larger primary image and the gallery from the Image Gallery that is controlled by Simple Image Gallery Pro displays below this primary image.

Is it possible to make the larger primary image part of the gallery, ie clickable to launch the image gallery? It seems odd from a users standpoint that they are completely disconnected and use two different modal popup systems.

Out of the box, this is not possible.
It coulf be possible if in your overrides you removed the 'modal' class of the link and pass the rel attribute to the link with a simple JS script.

I'm sure I'm barking up the wrong tree here but I hope this makes its way to those that can make a change, but don't you agree that from a site visitors standpoint, that it's strange and would be perceived as "less than professional" to have, out of the box, two different modal systems in the same group of images and that the primary image and the image gallery are in no way connected? I think the expectation of your site visitor, when they open a product page and see a larger primary image with smaller gallery images below is that they would click the larger image to enlarge it and could cycle through all of the images as a single gallery using a single modal system.

The K2 developers really should correct this to allow us to use the image tab if we're not using SIGPRO or the gallery tab if we are and when using the SIGPRO gallery, the first image of the gallery, or a designated image within the set, becomes the image used in all instances where the image from the image tab is now represented.

This would be nice for the K2 V3.
As for different modals, K2 uses Joomla!'s default modal (keep in mind that both J2.5 and 1.5 are supported.) while the extension uses a multitude of lightboxes.

It is possible to do this with a simple JS scripts on the overrides.
If you are interested I could make a gist in github.

I think that would be great! I appreciate the offer.

It will benefit the community as well.
I will post it here when it's ready.