ei only correpond to ii if it originaly is a long i - otherwise, it's äi. Baseldeutsch has traditionnaly ei like High German, but I hear often äi in younger generations.
careful: ie would be seen as iä, a diphtogue, not a long i like in High German.

ist is a verb form indeed pronounced isch in the whole country. I would love to be prooven wrong on that... if somebody know a place in CH with an other form, please express yourself :-)

-li dimminutives are not necessarily used the same way as High German or other languages with dimminutives. And all -li words don't mean small things.

I am not sure it is true that Swiss German dialects do not obey sets of grammar. They just have one set each. That's many.