>>7152761Dio says toki YO tomare and here's a picture showing it, よ being read as yo, 時よ止まれ."Wo" is grammatically incorrect because 止まれ(tomare) is the command form of 止まる(tomaru) and 止まる is an intransitive verb (these verbs usually describe a state change). Only transitive verbs (these verbs describe an object being acted on by a subject) can take the particle を(wo, the object marker). Now, the reason よ is used is because written Japanese will sometimes replace the subject marker with よ when being poetic so you'll often see this in poetry, songs, books, etc (whether this ties into Dio's rich upbringing idk). Much like in English when we write "O wind, blow!", Dio is saying "Time, STOP" basically commanding time itself to stop.
As for your second question, it's hard to classify all furigana (the writing next to the kanji) into one use but often manga will put furigana next to tough to read kanji or maybe kanji the reader may not be familiar with since most manga has kids or teens as a demographic. So maybe the reader can't read the kanji but they can read the furigana and go "oh I know that word". But like in my previous post, it can be used for loan words, double meanings (these are always fun and the most interesting use of furigana), etc. Sorry for the long post but I hope this helps you understand.
