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Decius comments on Logical Pinpointing - Less Wrong
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You can't talk about what the natural numbers are and are not without some form of set theory.
"0 is the only number which is not the successor of any number" requires set theory to be meaningful.
But you can talk about some of the properties they have, and quite often that is all we care about.
Also, the stronger your system is, the more likely it is that your formulation is inconsistent (and if the system is inconsistent, you're definitely not describing anything meaningful). I'm much more confident that first-order Peano arithmetic is consistent than I am that first-order ZFC set theory is consistent.
No. You can rephrase that as: "Every natural number is either 0 or the successor of some number".
What does "Every x" mean in the absence of set theory?
Enjoy A Problem Course in Mathematical Logic. Read Definition 6.4, Definition 6.5, and Definition 6.6 (Edit: They are on PDF pages 47-50, book pages 35-38.). It means that, within each model of the axioms, it is the case that every object in the model has the specified property. The natural numbers happen to be a model of first-order Peano arithmetic.
Let me ask you what "every x" means in first-order ZFC set theory. Answer carefully - it has a countable model.