1&3: A nuclear repository needs to be sealed to prevent radiation. A chemical landfill does not. People can see the waste inside landfills from afar and realize that it's nothing of value.

I work in waste management, and every closed surface impoundment I see reminds me of an ancient burial mound. But my point is more that while our civilization is around, we need to manage those sites, so we will also be able to manage a nuclear site. This post-apocalypse site is useless while our civilization exists.

1b: There are parts of spent fuel that are completely spent and unreusable; in fact, reprocessed uranium costs way more than just dumping the spent fuel and buying new uranium.

True, now. Which is I said we'd need to find an economicslly viable method, but assuming we will never is unlikely.

2: You are assuming that the future beings will only discover and somehow manage to breach the place before recivilizing.

Absolutely not. Even prehistoric humans figured out not to live near the malaria pond, not to eat the wrong berries and which parts of the animal make you drop dead. And if they maintain civilization, all the warnings are useless anyway.