As far as I know you can't set/read a string value with an exported string, you need to reference the string by name directly in your events. It might be possible with scripting, though I haven't tried that.
I would recommend you use a dictionary to store all your strings and values, then you can easily set/read them with a string. By splitting the text using tokentat based on spaces.
tokenat(strVar,0," ") will get you the string name
tokenat(strVar,1," ") will get you the valuation
tokenat(strVar,2," ") will get you the result
Then you can use dictionary.get(tokenat(strVar,0," ")) = tokenat(strVar,2," ")
Since you can't also inject a valuation string and need to directly write it in your event, you would probably need to do a separated check for each type of evaluation.
if tokenat(strVar,1," ") equals ("==") >
Dictionary.Get(tokenat(strVar,0," ")) = int(tokenat(strVar,2," ")) ? "correct" : "incorrect"
if tokenat(strVar,1," ") equals (">=") >
Dictionary.Get(tokenat(strVar,0," ")) >= int(tokenat(strVar,2," ")) ? "correct" : "incorrect"