Pluralizing Mubi biradical nouns

(Attention conservation notice: Kind of technical...)
NB: Updated with important corrections shortly after posting, following discussion with Marijn van Putten.

To better understand the Mubi plural system (following up on last time), let's start with two-consonant nouns of the form CVVCV and CVC (not CVCV, which has different default plural templates). Most of these take a CVVDvD plural with reduplication of the final consonant, with the vowels accounted for by the following correspondences:

• CeeDi > CaaDaD, e.g. gèébí "horn" > gàabàp (final consonants automatically devoice)
• CiiDi > CeeDaD, e.g. lìísí "tongue" > lèesàs
• CooDi, CuuDi, CuD > CooDaD, e.g. fùúdí "thigh" > fòodàt
• CooDo > either CooDaD or CooDuD, e.g. góoró "throat" > gòoràr; zòoró "tributary river" > zòorúr
• CaD, CaaD, CeD, CoD, CaaDo > CooDuD, e.g. fáaɓó "breast" > fòoɓúp

(No such nouns end in -a, -e, or -u.)

There seem to be no instances in the lexicon of *iiCa or *uuCa (the only case of iiCaa is in an Arabic loanword); nor are there cases of *aaCi or *aaCu, except in manifest loans like áarìt pl. àwáarìt "devil" (Arabic ʕifriit), with the odd exceptions of gíráakúmò "molar" and káarúmo "fingernail". We may thus assume that there is backwards spread of height from the short vowel to the long vowel preceding it. The latter may be mid but must not be the opposite height to the short one, with conflicts resolved by changing to mid and harmonising for frontness and roundness:

• ii > ee / _a
• uu > oo / _a
• aa > ee / _i
• aa > oo / _u

Generalizing the latter point, it also looks like roundness spreads backwards to a long vowel in a preceding syllable, i.e.:

• ee > oo / _o, u
• ii > uu / _o, u

On a quick glance through the lexicon, the only exceptions I can see to this are vocalised semivowels as in hàaɗáw, pf. héeɗû "knead". This allows us to reinterpret CeeDi above as /CaaDi/, and CooDo as either /CeeDo/ or /CooDo/. We thus get a nice distribution of allomorphs: -aD if the singular contains an underlying front vowel, -uD if not:

• *CaaDi > CaaDaD
• CiiDi > *CiiDaD
• CooDi > CooDaD
• CuuDi, CuD > *CuuDaD
• *CeeDo > CooDaD
• CooDo, CoD > CooDuD
• CaD, CaaD, CaaDo > *CaaDuD
• CeD > *CeeDuD

Now we can almost rewrite the rule rather simply to unite all these cases:

sg. CVD, CVVDv [+front] => pl. CVVDaD
sg. CVD, CVVDv [-front] => pl. CVVDuD

The problem with this reformulation is that CooDo < *CeeDo nouns take a plural in CooDaD, not *CeeDaD. Perhaps this is best understood as an effect of the deleted final vowel: the delinked /o/ relinks to the previous vowel. But if so, this only applies for rounding, not for fronting...

This information, missing from the published grammar, gives us some of the necessary background to understand where those pesky CuCooDuC plurals might come from... But that's a story that still needs work, to be continued later perhaps.