When an apostrophe is used to indicate a glottal stop, that is regarded as a separate letter and thus not an accent mark. However, a transformation of a consonant could be indicated by an accent, such as palatalization, so if there is such a thing as glottalization...

Ah, yes, Wikipedia tells me that glotallic consonants are represented with an apostrophe in the IPA.

U+0315 was added to Unicode in version 1.1, and is a combining diacritical mark. But I can't find the original proposal for adding it to Unicode.

...is it intended to represent the alternate form of the haček found in pairs like Ď ~ ď?

Yes. U+0315 is provided for compatibility with visual encodings that handled e.g. ď with a separate character from the normal haček. It's mostly useful these days as a component glyph for use in making diacritic composites for Czech and Slovak glyph sets. Note that although nominally a combining mark it actually requires an advance width, so is not usually handled with GPOS dynamic mark positioning.

The glottal stop, when so signaled using one fairly common orthography, uses the "raised comma," U+02BC. But remember, that's just one orthography. In Polynesian, the glottal stop is signaled with the turned comma, U+02BB, as in Hawai‘i. And in some languages, the glottal stop itself is used.

Note that although nominally a combining mark it actually requires an advance width

Where is that specified? Somewhere in http://unicode.org/cldr/utility/character.jsp?a=0315 ?

Thanks, John

That answers my question perfectly.

André