If anyone is interested, I am continuing to work to solve this dilema. In the MM_callJS and after the script to change the variable (myselector) contents, I inserted "document.write (myselector)". Interestingly, the variable contents had been changed but the change was not reflected to the rest of the code. I'm still working on the problem.
Through futher work, I've narrowed my focus to the "myselector" variable. The variable is defined in the head and redefined in the function. Within the function, I am not using "var". Though the variable "myselector" should be global, the changes to its value are discarded when the function closes. This indicates that it is functioning like a local variable in the function. After a bit of reading, I've not found the cause of the global/local discrepancy or a fix for it.
I hope that someone reads this and guides me.
I thank you.