It looks like my "New Fowler's Modern English Usage" - R W Burchfield, Oxford 1999 has decided to sadly omit the discussion shown above.

He says "No other grammatical discussion has so divided the nation since the split infinitive was declared to be a solecism in the course of the 19C."

There is a wonderful section with examples from the 13C to today with writers from Chaucer to Peter Carey, Iris Murdoch and Kingsley Amis.

Not surprisingly, his final quote from "The Spoken Word", Burchfield 1981, wins the day "Avoid splitting infinitives wherever possible, but do not suffer undue remorse if a split infinitive is unavoidable for the natural and unambiguous completion of a sentence already begun".

Sounds like (5) to me!