A question about punctuation ... not relevant to "Tatoeba day"

I actually have a few questions:

1. The "em dash"

In typesetting (and in html) there are two types of dash: the "n dash", which usually appears between numbers (eg, 1-9), and the "em dash" that's longer and usually appears between words, generally to indicate a pause (colon or semicolon) or parenthetical sentence. I mean, for example, "late againーwhat a surprise!".

When I use HTML/XML for this kind of thing, I just use the "mdash" HTML glyph, but here I've ended up just switching over to Japanese input mode and entering a hyphen, which get rendered as in my example above.

So my question is whether this is a good/correct way to get that symbol into a sentence?

2. Localisation of quotation marks

In English and Irish (which are my main languages), it's simply a case of using the double-apostrophe key, whereas other languages (eg, Spanish, German) use different characters to delimit quotations.

My question is whether English-style quotes are acceptable in general, or whether the localised text should always follow local rules (eg, 「」 for Japanese) ?

This is more a question about any automatic tools that process the data rather than what I should be typing in (ie, I guess I should always use the correct language-specific punctuation).

3. double-width space characters

If I make a mistake and include a double-width space character (due to being in Japanese input mode) instead of the usual space, will it mess up the value stored in the corpus? Or are these automatically converted to regular spaces (and coalesced, should I hit space twice)?