The possibility of "unlimited" Alert Summary is going to create a problem here and I think it necessitates doing the data prep outside of Tableau. Did you find a solution for this already? i see this was posted a few weeks back and maybe you found a solution worth sharing. I'd go back to the system creating those rows and see if I could modify it to create the desired output.
Thanks for your reply. In the end I did just bite the bullet and re-work the data flow to include a pre-processing step, I didn't find a way to fix this.