I always thought avocado was the Spanish word, and guaca along with guacamole (guaca + mole meaning sauce) was the indigenous word.
https://en.wiktionary.org/wiki/guacamole#Spanish
> From Classical Nahuatl āhuacamōlli, from āhuacatl (“avocado fruit”) + mōlli (“sauce, something ground”).
Avocado -- mid 17th century: Spanish alteration (influenced by avocado ‘advocate’) of aguacate, from Nahuatl ahuacatl.
Guacamole -- from an Aztec dialect via Nahuatl "āhuaca molli", which translates to "avocado sauce/concoction".
I think you mean "influenced by abogado ‘advocate’".
The classical /w/ from Latin obviously became /v/ in a lot of romance languages. /b/ and /v/ merging to create [β] I think was particular to Spain and pretty old. But I think I also read that in many places vulgar Latin /b/ had a [v] allophone, but still distinct from /w/ (orthographic v) which I guess evolved into /v/ outside Iberia with the old [v] sticking around as /b/.
Here is one of the top google hits from an old saying about this: "beati hispani quibus vivere bibere est" --http://en.antiquitatem.com/felices-hispani-quibus-vivere-est... -- this seems to put the b, v merger at the first century AD and have a lot of quotes about Iberians doing it it from the ~1500s, blaming it on Basques.
By the way, in my amateur opinion the much more head-spinny Iberian sound change was the creation of modern /x/ in words like "Quijote" or "México". It would seem that the Spaniards conquered the new world with those things as [ʃ], transcribed a bunch of Mexican place names with that, then later changed the sound to [x], and brought the change back to the colonies. It is interesting to me that such a pronunciation change in relatively isolated populations can still be held on both sides of an ocean, and not have some other form just end up as one side's peculiar accent, as occurred with, say the evolution of the [θ] sound.