Am I right in thinking that we can’t make these arrowheads (which indicate dependence on a prototype) invisible in Map view ?
Sure you can — make prototype links invisible. In fact, invisible prototype links are the default
The missing piece of the puzzle was that in the absence of a link type with the name
'prototype' in the document inspector list, we get a default black color, and no immediately visible route to editing the properties of prototype links.
(I had previously chosen the
Delete all unused link types options)
Once I had added a new link type, and given it the magic name “prototype”, all was in reach : -)