Whether to ADD .com to rel-alternate or ADD .com AND REMOVE X-default?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  Micah Fisher-Kirshner: Will throw this one to the community around HREFLANG implementation:
  
  IF your X-default is the .com (US), should you also include en-us as .com (US) as well?
  
  I've noticed that in GWT it doesn't seem to recognize the return tag back to the US (ie: we reference Canada URL, but the Canada URL goes back to the US via X-Default which GWT doesn't seem to realize).
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  Masatake Wasa: Oh, that's a good question: I'd say no (in the sense that there is no need to do it), but I doubt it would do any harm. Having said that, I have no clue, and it'd be good to hear what others have to say. My reasoning - faulty as it may be - is that x-default should cover all other languages other than those specifically mentioned in hreflang.
  
  So let's say there is a .com site that targets everywhere except Canada for which there is a .ca site in English (let's leave the French-speakers out in this discussion), then the only swap that needs to take place is in the English results in Canada achieved through hreflang="en-ca". Elsewhere, whether it's in the US or say Mexico or even Australia, you'd want people to end up on the .com site, and x-default should cover all of that.
  
  [Added] Alternatively, instead of x-default, it may make sense to have a generic en (rather than specifically en-us) covering all English queries except en-ca? After all, the content is in English and aimed primarily at those who are English-speakers searching in English: you wouldn't expect the site to appear for Spanish queries in Mexico.
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  Micah Fisher-Kirshner: I'd like to avoid adding a whole other sub-section with /en/ :)
  
  Ideally it'd be nice if I could just say: "Hey, if you don't speak the languages in the countries we have, then go to our main English (.com) website. And if you're in the US and speak English, also go to the .com site"
  
  That's what I was hoping to get with the X-Default on its own.
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  Masatake Wasa: Trying to think logically (which is a bit difficult for me at the moment for some reason), x-default should achieve that? It suggests to me that the default for all language-locale is example.com, except for en-ca: example.ca will be shown in search engine results conducted in English in Canada, in the position example.com would have otherwise appeared.
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  Micah Fisher-Kirshner: Yeah, that's what I would think too, but in GWT, it's saying I'm not reciprocating the love with the return tag, even though it's noted in the X-default section.
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  Masatake Wasa: Hmm ... That seems odd. I'm using x-default in one occasion (so it's hard to say based on one example), and it's a three-way split (en, ja, and x-default), and it hasn't come up as an error on GWT. However GWT is claiming that there is no reciprocity on some pages (en and ja, no x-default involved) despite the fact that there are reciprocal and self-referring hreflang links, and as such should not be counted as errors.
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  Micah Fisher-Kirshner: Yeah, I've noticed that they are behind with noting the reciprocity sometimes (if added later), but I'm just trying to figure out at this point whether to ADD .com to rel-alternate or ADD .com AND REMOVE X-default.
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  Edwin Jonk: From the expert panel in this weeks SEO Questions hangout on air on 00:03:17 into the YouTube video: https://dumbseoquestions.com/q/whether_to_add_com_to_relalternate_or_add_com_and_remove_xdefault +Micah Fisher-Kirshner
  
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