gadahl Okay, after looking at the mixer you were using and how long you mixed it for it is absolutely possible that you mixed it so much that you denatured the protein. That would explain what happened to a large extent, the denatured protein no longer has the same bonds and it will not help hold together the product any longer. So, yes you can over mix meat.
Another possibility is that the homemade mix you used did not contain the correct percentage of salt. I am not saying your mix was no good, some people have home recipes for mixes that work wonderfully but salt plays a key role in protein extraction as it allows meat, fat, and water to bind together more efficiently. In almost all seasonings that are going to be used on a cured product salt will be the #1 ingredient. For example, even the Reduced Salt Frank & Weiner Unit has salt as the #1 ingredient.
It is also possible (though unlikely considering how much you mixed it) that your chunks of added fat simply did not come into enough contact with the proteins. If any large chunk of fat existed, even if they were completely surrounded by lean meat they would still render out of your product during the smoking process. This would only really come into play if you added your fat and your lean separately, this most often happens when making a venison summer sausage or snack stick where pork fat is added separately, but again I don’t think this was your issue. A good way to prevent this from happening would be to grind added fat at the same time as your lean.
Your water and carrot fiber ratios look fine to me.
Here is one other possibility, the mixer you used might have caused so much friction that it raised the temperature beyond optimal levels? I know this can happen if you run the product through a hot mixing plate, I’ve never seen it happen when mixing but I’ve also never used that type of mixer. However, I also know you would be far from the only person to use this type of mixer.
So, all in all, I think that the most likely issue is that when using a meat mixer like the one you did that 8 minutes is just way too long and it denatured your proteins preventing them from binding.
My recommendations would be that you try again with our seasoning (correct amount of salt) use a binder (sounds like you already are!) and that you mix for a shorter amount of time if you are going to use that Kirby again, or use another hand crank mixer (like the Weston 20 lb for the recommended amount of time. I think once you do this your wife will stop seeing the horn in the middle of your head and you will go back to being a hero! I can’t help you with her muttering about the mess though but hey at least she isn’t a vegetarian like mine was for a few years, we have recently welcomed her back into the fold of carnivores!