(the original post had a random word converted into a link to some spammy website that had nothing to do with the question or Notepad++, hence the original poster – now banned for spamming – is not going to get credit for the solution.)

It doesn’t solve the underlying problem, but -multiInst does seem a reasonable workaround for someone who finds themselves in a multi-desktop environment. But I don’t know that it would solve Explorer > Double-Click or Explorer > Open With Notepad++ – I don’t know whether, if there’s an open Notepad++ in the current desktop, whether that instance will get preference over another Desktop’s instance, or whether the OpenWithNotepad++ will just always go to the first instance, regardless of one being open on the active desktop.

it might be something to experiment with, until such time as a solution is implemented.

No solution.
Notepad++ 's limited development team hasn’t tackled this particular problem yet.
There are already official “issues” open for it.