The differences in regional accents between say local Dublin English and the Supra regional Dublin accent (i.e. without even leaving the city) are reasonably large. I dunno what italian dialectal differences is, but Englishes are often very large city.
I'm not saying it's impossible; Trainspotting, for example, is mostly written in phonetic Scottish english. Many english speakers find it difficult to read though, for precisely that reason…
No, not at all. Several Italian words have regional variants, and they are simply spelled differently.
When you are used to a phonetic alphabet, you never wonder how to write what you say.