In the US, they're interchangeable in almost all circumstances. I use "further" to talk about distances all the time. The only difference here is that "further" can also mean "more" in collocations like "further news" or "further updates."

There is no difference, just two alternative spellings/pronunciations for the same word. Many words have alternative pronunciations. The most common word in the English language, "the", is such a word. Sometimes people pronounce it "thee" and sometimes "thuh". Fortunately, there is only one spelling for "the".

We usually use farther with literal real distances. The wind blew the paper bag farther down the road. China is farther away than Japan. And use further with figurative things. I want to go further in my career. Etc