"있을지 없을지 저도 잘 모르겠어요." => I am not sure myself whether it's going to be there or not. Without 도, it sounds flat and dry, a simple "I am not sure whether ...". 도 adds the sense of "myself" and makes it smoother, like "(I understand you/they are unsure, and) I myself ...". * to be sure: - -(이/가/에/것이/...) 틀림없다 / 분명하다 (clear - but often used like "sure"). informal - -이/가 확실하다 (certain); -을/를 확신한다. more formal * to not be sure: - -ㄴ(지/지도/가) (잘) 모른다 [모르겠다] / 확실치 않다. - -이/가 불확실하다 (uncertain); -(라고/을/를) 확신할 수 없다 / 확신 못한다.

도 can mean "too" or "either". In the case of this sentence it means "either" since the verb means "don't know". 겠 doesn't necessarily mean assumption or guess. It can also just be attached for formality. In other words, 몰라요 and 모르겠어요 have very little difference; the difference being politeness and not nuance. It's very subtle and hard to translate into English. But, I wouldn't put too much weight into that suffix in this sentence. "I don't really know either if there is or isn't" I don't know the context of the sentence, so that is my best translation. Depending on the context, this sentence could refer to something in the present or something in the future. Hope this helps!

I'm not sure what's your question. 도 means "also" 저도 잘 몰라요. means "I don't know either" (literally "I also don't know") 저도 잘 모르겠어요. means "I guess I don't know either" or "I'm not sure either" 있을지 없을지 means "if it will exist or not" 거기 있을지 없을지 or "if it will be there or not" (거기) 있을지 없을지 저도 잘 모르겠어요. means "I'm not sure either if it will be there or not" "to be sure" means "확실하다" , "분명하다" , "명확하다" "to not be sure" means "불확실하다" , "불분명하다" , "불명확하다" , "잘 모르겠다"