- What would be an example of "[h] [...] as an emphatic, interjected sound"
They mean interjections like a laughing "haha" as [ɦaɦa] or [ʕaʕa], or [ha:ʁ] as an expression of frustration.
- I found this example: maintenance /mɛ̃tənɑ̃s/ --> [mɛ̃ʔənɑ̃s], is that okay? Why does this happen, why does glottal stop occur between vowels?
They meant a glottal stop occurring between two vowel phonemes, to break the hiatus. Something like aorte /aɔʀt/ pronounced as [a.ʔɔχt̪]. This can also happen between an article and words beginning with an "aspirated" h: la haute /lao:t/ [la.ʔoːt̪].
This isn't a very common way to break a hiatus in contemporary French, and mostly occurs in overenunciated speech (try listening for it in a dictation).
- What does it mean: that neither [h] nor [’] occurs consistently with such words?
"Such word" refers to the "“h-aspiré” words" in the previous sentence. The author means that no speaker(*) will consistently realise all the words that begin with an aspirate h (and no other word) with either [h] or [ʔ].
What actually happens is that French has a variety of strategies to break a hiatus, and that every speaker will use all of them, somewhat randomly, whether this hiatus occurs because of an aspirate h or no: We can leave the onset empty and the vowels remain in hiatus; we can insert a glottal stop; we can insert a glottal fricative; or we can insert a glide.
Thus, any speaker might pronounce "l'aorte" or "la haute" in any of the following ways:
[la.ɔχt̪] [la.oːt̪] (vowels in hiatus)
[la.ʔɔχt̪] [la.ʔoːt̪] (glottal stop insertion)
[la.ɦɔχt̪] [la.ɦoːt̪] (glottal fricative insertion)
[la.wɔχt̪] [la.woːt̪] (glide insertion)
And chances are that any French speaker that listened to any of those would hear the same sound between /a/ and /ɔ/ or /o/: nothing.
(*) There are actually some French varieties whose substrate possessed a /h/ phoneme. Some speakers of those varieties might consistently realise aspirate h words with a [h], for example speakers from Liège in Belgium influenced by Eastern Walloon, a language with /h/. The vast majority of French speakers don't speak that way.
- Can you think of an example for "Depending on style or on context, either of these sounds ([h] or [ʔ]) may occur word-initially or as a hiatus breaker where there is no “h-aspiré” involved"
I just gave you some with "l'aorte". [h] or [ʔ] can also occur at the beginning of any word that starts with a vowel, especially if it's at the beginning of an utterance or said insistently.