The complex workarounds discussed in this thread have me totally baffled
I created a simple test epub containing the various special space characters - no fancy formatting - and have no problem at all viewing them in the calibre Viewer nor on my Kobo. I tried various fonts (including Georgia and Times New Roman) in both apps and did not see the reported 'black boxes' with any of them.
I
did use the calibre Editor, not Sigil, to create the epub so all named entities have been translated to the relevant unicode characters. Perhaps this is why I have no problems.
Here's a screenshot of the calibre Viewer using both Georgia and Times New Roman. I could post the test epub if anyone's interested.
N.B. All paragraphs are left-aligned so there's no space-adjusting involved.