Success! This is on the online Kindle Previewer which is supposed to be the definitive look at what will display in e-print. Now the second time I exported to epub3 I didn't get that warning on Pubcoder's validation for the declaration etc...
What is that about? I'll let you know if it comes up again. Oxygen Author doesn't seem to mind it, though I didnt read al those warnings it had. Mostly they are about extra fonts and other stuff that doesn't matter to validation.

It validates with warnings. I will give you more of an update later. Have to go now.

Got this on export just now.

Thanks Angelo!

Kindle Previewer has lied to me before but it displayed as shown here in a published ebook. KP on the desktop does not show the error to you, only when you upload to Kindle online.

You can get an em dash to display?

Mac. sorry

Got this from a guy on Facebook epub production page,
"I had similar problems in other programs. Typically it is caused by the wrong encoding. Each character can be expressed by more than one entity. Unfortunately, not all applications can interpret every entity type. Some entities are all numbers, such ӄ while others can be expressed with letters, such as   You will find that most applications support the first, but not all support the second. All letters and character also have a hex code. Sometimes the solution is simply using the supported entity, in others you have to declare the encoding at the top of the document. Also, be aware that “plain text” documents may not support non-ASCII character sets. In 1995, most computer apps and file types switched to Unicode which supports all TrueType character sets. But you will have to check to see if the app you’re using supports ASCII or Unicode. I believe ASCII supports only 256 characters. I had many of these issues back when I was working with XML."

Here's what I see with an em dash after export from pubcoder and upload to Kindle previewer. After the word 'luck'.