There is so much misunderstanding in your question that I don't think you can get an answer in its current form.

Spanish is a romance language, meaning that its descended from Latin. It's not some kind of 'offspring' of Latin and whatever else was in Spain. So this talk of "retaining" elements of Iberian languages is incorrect. Language pronunciations can be influenced by cross-language interaction in bilingual populations, but that's unlikely to be a sufficient explanation for why Spanish, or other languages, sound the way they do.

French has celtic pronunciation

I don't know where you've got this idea from.

and nasal elements,

Latin had nasal elements. Final m sounds were probably nasalised, as were gn consonant clusters.

it makes sense for Italian to be close to Latin as the heart of the Roman Empire.

It makes popular sense, but the mere geographical location of contemporary Italian doesn't make it any stronger a successor to Latin than the other romance languages. Geography doesn't "make a case" for Italian's "purity" in respect of Latin.

The problem with your question is this: the idea that Spanish sounds relatively closer to Latin than other Romance languages only appears to be the case to you. It's not an empirical fact.

Phonologically, Sardinian is probably closer. Sardinian has remained relatively conservative in its sound-system.