Mark, thanks, very illuminating -- and I thought this had given me the key to solving the problem!
As you note, the parent "imported spreadsheet" note takes on the "*Entry" prototype I had created, but that prototype did not include an On-Add action also setting children to $Prototype="*Entry" as well. So I thought that if I added that setting as an OnAdd action, all would be well.
And it works -- part-way! Now all the imported spreadsheet entries nicely have the *Entry prototype. But the importing process over-writes (and thus breaks inheritance) for their Key Attribute list. So it's still necessary to reset that to default, and then re-imprint it with the right values for that prototype. Easy to automate, now that I understand the underlying process. Appreciate it.