Can we also use "συμπαθεί" (he/she likes)?
Συμπαθώ is used for liking someone not something, in a friendly way.
Why not "You like the orange?" ?
Because normally, in English, that's not how question structures work. :P I am aware that this is also a structure used in speech, but in text, it would be better to stick to the rules, so that we don't end up with a gazillion alternatives.
I notice that the sigma is sometimes pronounced sh. In this sentence the first sigma is sh but the second is s: shou aresi
I wouldn't say there is a difference in the pronunciation of σ in different words. There might be some regional variations, but technically, it's always pronounced the same way.
You might perceive it that way because the greek s sound is a retracted alveolar one, which is basically the middle point between "s" and "sh".
How would we translate: "You like the orange"?
It's the same, just without the question mark at the end.