No hint for unreality? On the contrary, nobody's leaving, there's just fear, so the leaving is at this stage perfectly unreal. That's when other languages use the form indicating unreality, usually called subjunctive, or jussive in Esperanto. I'm still confused. But it's true, an internet search gives me mostly 'mi timas ke ---os' and 'mi tesperas ke ---os', and very few examples with -us. So it is correct alright. I'm just trying to figure out why.
If you'd use "forlasus", then you should use timus as well.
"Mi timus, se li forlasus min" - note, that you can't use "ke" then, you need "se" - I'd be afraid, if he'd leave me.
You use this sentence if you are certain or if you are not certain! There is no difference here in Esperanto.
And we are talking about a future event here, when we use this sentence.
Well if it's anything like the others are saying, then it's like using abandonar (Spanish) and laisser (French). And also, if you think about it, For means away or out if I remember correctly. So it's like "to leave away/go away/depart" etc. I would think of it almost as a sortir vs partir kind of situation as well. But this is also in it's own language and culture so you could even just disregard these parallels! :)