I don't think it is that spillover was set, it was off (but even when it is on, I've been getting the same results, possibly because it is a 'paste' instead of a note entry).
It is more that, in one instance, being at the beginning of a measure and executing the command the way I set it up, with, albeit excessive code (Wasn't able to get that shortened version you whipped up to work on first try), is different from when the top measure already has a note in it, and that is because it [the program] was told to move-cursor-left, which would be the appropriate action if there was already a preceding note in the measure. So basically, setting up the conditionals to run or omit the two left-moves if cursor is at beginning of measure. Although something like that would be simple for someone like you, I would have to ponder over it much to get it going. There are other instances where script runs into little hiccups, and the solution must [partly] be in asking certain questions about the measure contents, where the cursor is, etc.
The other thing that might be useful, is to get user input of the duration to be required for the new note on the upper staff, if that is possible (I think you mentioned note duration requirement as a possibility, maybe that isn't what you were saying though).
Anyways, thanks for reading my rambling. And just because I don't respond adroitly to your code examples, that is mostly because it is taking a little while to sink in, but it is good nevertheless.
Rain
withAndWithoutSpilloverSet.jpg