Getting closer is always cause for celebration.
From what I have been able to deduce Excel doesn't like or can't cope with "dead-ended" references and has no internal mechanism to close references opened by remote applications. The new Word / Excel toolkit that NI has is much better at handling Excel. I haven't used it much but I have looked at every VI inside it and they seem to follow the rules of my previous post.
Its OK to keep, what I call major references open, which I classify as Excel._Application, Excel._Workbook, Excel._Worksheet, and possibly Excel._Sheets (I don't remember this exactly, and its been a year or so since I was majorly involved in excel). But make sure you close them in the reverse order you opened them. So you would close Excel._Sheets, then Excel._Worksheet, then Excel._Workbook and then Excel._Application. Clear as mud? For Minor references such as , cell, range, etc I would open references, do my work and then close them.
I'll give an example. Say that the Excel application references are open and a worksheet is open.
Say this is SubVI A
Open reference 1
Use reference 1 to open reference 2
Do something with reference 2
Close reference 2
Close reference 1
So this VI would contain to Open references and 2 Close references
Continue on to next subvi.
I have attached an unlock diagram for NI toolkit. Hope they don't mind. It a very poor jpg but I wanted the size to be small.
What you can't see is that they keep those "major references" open and this is a subvi that does something. Notice the order that the minor references are opened and closed.
I'm not sure if it will be a problem to perform several simultaneous operations or not. I would think that as long as you close the references in the proper order it would be ok. I don't think we did that.
This sort of thing is something that I would make sure that I would use lots of subvis and lots of error handling.
Hope this helps...some more...
Dean