Bare genitives: written vs spoken forms

Page 1595 of the CambridgeGEL reads

In writing the bare genetive has the form of an apostrophe at the end of the word: dogs’. In speech it has no realisation at all [...]; an optional bare genitive is found in certain types of proper names, where it is more likely in writing than in speech.

It's obligatory with plural nouns ending in s, regular or irregular. Nouns like species which have identical singular and plural forms with final s take it in the singular as well as the plural, and in writing this will apply to nouns like chassis too.

The bare genitive is the only possibility in fixed phrases with sake : for convenience’ sake has a spoken /s/ but not written s.

As is clear, there are several contradictions in this page.

According to the text, the singular species (ˈspiːʃiːz —some speakers pronounce the singular with -ɪz, the plural with -iːz) also takes a bare genitive, species', unlike the example given by the author in the same page quiz's (which follows the usual rule). Therefore, one cannot never know whether species' refers to the singular or plural.

Furthermore, chassis' for both the singular /ˈʃæsi/ and plural /ˈʃæsiz/; why not the usual rule for the singular one chassis's /ˈʃæsiz/?

Must bare genitives be divided into spoken and written forms? Otherwise, what characteristics define bare genitives?

The CGEL example is misleading. The authors have chosen not to emphasise that “convenience” ends in a sibilant - the /s/ sound: “for God’s sake” is quite normal as “God” does not end in a sibilant. Further, they make an unsupported statement “for convenience’ sake has a spoken /s/ but not written s.” This cannot be justified. It is important for students to understand that there is no final authority in English and there is no justification for the absolute claim.

OED

II. for the sake of (also †for sake of); for (one's, a thing's) sake. In the latter of these forms, the word which precedes sake is a possessive (noun or pronoun); but down to the middle of the 19th c. the 's of the possessive of common or abstract nouns was very commonly omitted (doubtless owing to the difficulty of pronouncing the two sibilants in succession) […]. The omission of the 's is now obsolete, but it is still not uncommon to write for conscience sake, for goodness sake, for righteousness sake, etc., without the apostrophe which is ordinarily used to mark the possessive of words ending in a sibilant.

That latter remark is now, itself, outdated and, at least, the apostrophe should be included – the s is usually optional in words ending in a sibilant.