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Regular sound changes result in the accusative and the ablative cases merging to the same phonetic form in the first-declension singular and second-declension singular. Latin /am#/ and /aː/ both go to Italian /a/; in fact, this also merges with the expected reflex of the Latin nominative ending for first-declension nouns. Latin /um#/ and /oː/ both go to Italian /o/,

Regular sound changes result in the accusative and the ablative cases merging to the same phonetic form for a large number of third-declension masculine or feminine nouns: those with accusative singular -em and ablative singualar -e. (Latin /em#/ and /e/ both go to Italian /e/--although in some words, like città < cīvitātem, final /e/ was later lost).

Regular sound changes result in the accusative and the ablative cases merging to the same phonetic form in the first-declension singular and second-declension singular. Latin /am#/ and /aː/ both go to Italian /a/; in fact, this also merges with the expected reflex of the Latin nominative ending for first-declension nouns. Latin /um#/ and /oː/ both go to Italian /o/.

Regular sound changes result in the accusative and the ablative cases merging to the same phonetic form for a large number of third-declension masculine or feminine nouns: those with accusative singular -em and ablative singular -e. (Latin /em#/ and /e/ both go to Italian /e/--although in some words, like città < cīvitātem, final /e/ was later lost).

However, Italian plural endings are not consistent with the expected outcome of Latin first- and third-declension ablative plural endings. Italian plural -i is consistent with the expected phonetic outcome of the Latin second-declension ablative plural ending /iːs/, but also with the outcome of the Latin second-declension masculine nominative plural ending /iː/. Italian plural -a, found on a minority of originally neuter nouns, is consistent with the Latin nominative/accusative but not ablative form.

This answer focuses on the results of phonetic sound changes applied to Latin forms, and does not address the topic of changes in the use of ablative case in different stages of Latin.

But in the plural, Italian forms such as buone "good" (f. plr.) and cani "dogs" are not consistent with the expected outcome of Latin first- and third-declension ablative plural forms ( bonīs and canibus). Italian plural -i as in buoni "good" (m. plr.) is consistent with the expected phonetic outcome of the Latin second-declension ablative plural ending /iːs/, but also with the outcome of the Latin second-declension masculine nominative plural ending /iː/. Italian plural -a, found on a minority of originally neuter nouns, is consistent with the Latin nominative/accusative but not ablative form.

Therefore, it would certainly be untenable to say that Italian forms come entirely from the Latin ablative, to the exclusion of the accusative (or nominative). However, there seems to be legitimate debate about whether it's more accurate to say that Italian forms come primarily from the Latin accusative, or that they come from a merged "oblique" case that developed when the regular sound changes mentioned above caused the Classical Latin case system to become unstable. See Gaeng 1983 for an argument for the second way of putting it.

This is often identical to the expected phonetic outcome of the Latin ablative form, because the accusative and ablative singular frequently differ only by the presence or absence of final -m and/or the length or quality of the vowel in the last syllable, and due to regular sound changes between Latin and Italian, word-final m- in any unstressed syllable was eliminated and many vowel length and quality contrasts were lost.

However, Italian plural endings are not consistent with the expected outcome of Latin first- and third-declension ablative plural endings. Italian plural -i is consistent with the expected phonetic outcome of the second-declension ablative plural ending /iːs/, but also with the outcome of the masculine nominative plural ending /iː/. Italian plural -a, found on a minority of originally neuter nouns, is consistent with the Latin nominative/accusative but not ablative form.

This is often identical to the expected phonetic outcome of the Latin ablative form. In the singular, Latin accusative and ablative case forms frequently differ only by the presence or absence of final -m and/or the length or quality of the vowel in the last syllable. Due to regular sound changes between Latin and Italian, word-final m- in any unstressed syllable was eliminated and many vowel length and quality contrasts were lost.

However, Italian plural endings are not consistent with the expected outcome of Latin first- and third-declension ablative plural endings. Italian plural -i is consistent with the expected phonetic outcome of the Latin second-declension ablative plural ending /iːs/, but also with the outcome of the Latin second-declension masculine nominative plural ending /iː/. Italian plural -a, found on a minority of originally neuter nouns, is consistent with the Latin nominative/accusative but not ablative form.