1) I would say there is no objective way to determine whether an action is good or bad other than its effects. However, thoughts are also a form of action, so your intention still matters. The actual spit in the juice may not have any effect and therefore not be bad. But I think whenever you act on an intention, the 'mental pattern' that gave rise to that intention is reinforced.
So if you spat in their juice out of spite, say, the action would reinforce your mental 'habit' of spite, which would eventually cause suffering to you and/or others. The effects are not obvious or short-term, but they are there.
If you truly had good intentions somehow when spitting in their juice and they didn't notice, I don't see anything wrong XD
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2) People are definitely capable of acting out of true selflessness, although it is true that many times people think they are being selfless when they are really after something.
It may seem that people are not capable of 'real' selflessness when you think about it, because it's not possible to be truly selfless if you're thinking about how to be selfless. If you're thinking about how to not be selfish, then you are being motivated by a selfish desire to not be selfish, and therefore are selfish. Yeah, hopefully that makes sense. The key to true altruism is non-self-consciousness, which can't be done if you are overthinking everything.
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3) I think in practice there is no reality other than one's experience. Nobody ever experiences the past or future, only the present moment; therefore only the present moment is real. What we call the past is actually only our memories - happening in the present. So time is only a mental construct and there is no way to prove otherwise. We can choose a theoretical model of the Universe as being billions of years old or a few days old, depending on which model does the best job of explaining and predicting stuff, but a theory is just a theory and only exists in our minds.