First thing: I have no idea what your three checkpoint actions are. I couldn't find the checkpoint action in the KM editor's action list, or in KM's online documentation, or on the internet (!) I'm not sure that's important, but I had to mention it. Are you using some future, unreleased version of KM? 
Second thing: I actually used the exact code that you showed me in my macro which I posted above. So I think that means you didn't understand my issue. That's not what I meant when I said, "KM doesn't have a way to assign a variable when the name of the variable is stored in a variable." What I meant was you can't do this:
Set Variable A to calculation 1
Set Variable B to string "A"
Set (variable whose name is stored in B) to calculation 2
How would I express that code in KM in three statements, and get the variable A to end up as a 2 after these statements executed?
What you showed was this:
Set Variable A to Calculation 1
Set Variable B to string "A"
Set Variable B to (contents of variable whose name is stored in B)
Your code sets B to the contents of A. My desire is to get KM to set the variable whose name is stored in the string B to be changed to some value. That's very different. And I was able to achieve that in my AppleScript code above. I'm wishing that KM had an action that did this directly.