@David-TP - We finally have a solution to the problem. It's not the most elegant, but it does work. Here is a snippet of an email I received from technical support:

"Due to the number of clients is large (75), we suspect that Deco's memory or CPU usage is too high, which may cause the failure to add Connection Alert entries. Please try to power off one or two Satellite Deco to reduce the memory usage of the Main Deco, and then try to add a Connection Alert entry again to see if it works."[sic]

So, what I did is cut the power to my house with the exception of my modem and main Deco unit. Sure enough, I could (and did) create as many known device connection alerts as I pleased. Unfortunately, as soon as I added my devices back to the network I was unable to add any additional connection alerts. However, the connection alerts I did add are working, albeit slowly. 

This does make sense to me because anytime I checked my Deco network via the web portal it showed CPU usage pegged out at 100%. I've reported my findings to the support team and hopefully they will find a more user friendly solution. In the meantime I wanted to update you so that there is some solution for the next person who may encounter this issue.