In my experience it's pretty hard to get sequence numbering right in  amerge
whatever you do. For starters I would personally always try to avoid LISTNUM
anyway since it relies on Word understanding exactly where each list begins
and ends, and there is no way using field codes to tell it. You could try
doing your numbering based on SEQ fields (or perhaps even using { = } fields
to calculate the numbers (and \*Arabic etc. to get the correct format). But
I suspect that in that case what will cause problems is the IF field. If so,
you can try "inverting" the IF and INCLUDETEXT so that you have something
more like

{ INCLUDETEXT "{ IF { MERGEFIELD Month } = "APRIL" "C:\\SPRING"
"C:\\EMPTYFILE" }" }

but even then you may run into problems.