Spanish: -zco/-zo/-go in -cer or -cir verbs

[El Ganador]

Disclaimer: This question has been discussed to an extent here: merezco, conduzco (zc) / cruzo, venzo (z)
However, it is an old thread and I think that it does not abide to the newer forum rules for its location; it may be in the incorrect location (if a moderator disagrees, please merge the threads or do whatever seems correct)

What is the reason for which some verbs in the first person indicative end in -zco and others in -zo (and there exist the -go form, undefined form, and yacer's form as well)?

Examples of each (please tell if there are more/any is wrong):

-zco:
conducir -- conduzco (and all other verbs that follow this form)
agradecer -- agradezco (and all other verbs that follow this form)
placer -- plazco (and all other verbs that follow this form)
lucir -- luzco (and all other verbs that follow this form)


-zo:
mecer -- mezo (and all other verbs that follow this form)
fruncir -- frunzo (and all other verbs that follow this form)
escocer -- escuezo (and all other verbs that follow this form)

-go:
hacer -- hago (and all other verbs that follow this form)
rehacer -- rehago (and all other verbs that follow this form)
decir -- digo (and all other verbs that follow this form)
predecir -- predigo (and all other verbs that follow this form)

non-existent form:
balbucir -- no corresponde (and all other verbs that follow this form)

-go, -zco, o -zgo:
yacer -- yago, yazco, yazgo (and all other verbs that follow this form)

 

[S.V.]

Hello. You really want to identify 'core' verbs with a reason, to have a starting point.

ᴄᴏɢɴᴏsᴄᴏ: OSard. connoschian, conoskimus, conoscat (Port. conheçam, conhecemos, conheça)
ғᴀᴄɪᴏ: OSard. faget, faker, fager, fakites, fakiant, but fatho380, faço66, fazo20 (Port. faço, façam, etc.)

Notice this is similar on p. 269 (Fondow 2010), in that 'both' outcomes (tenga / tegna) are found within Italy. Or that table on p. 148 (MOc. veni, venga, MIt. vengo, venga). This is, an old process with variation, in which others in the periphery are pulled by core verbs. That fatho also has a match in pithinnu, pikinnu 'small'. I believe they don't agree on a reason for -sco > -zco.

On p. 131 you see one reason for the spread of -go (or -co), "complex alternation (‘duality’) in the paradigm, e.g. *teñ-o, tien-e-s." T’vegn’ a visité, bela burnetta (Canzoniere italiano, Forlivese, 'Venho visitar-te, morena linda'); Cal. tiegnu fame (733, 'tenho fome').