(0) It does not. It has a GC, and all features you'll likely use are memory-safe. You can of course use unsafe methods and raw pointers, but those are documented that they are unsafe and are usually only used for writing bindings to a C library. Macros for example are evaluated and constructed at compile-time, so no undefined behaviour can come from those. Most methods have different behaviour. For example, one method may throw an exception, another return an error, and another simply return Nil. You know if you are using `hello!` `hello?` or `hello`, so no undefined behaviour there.
(1) Not at all, look at the source implementation of their language implementation. For example, the lexer: https://github.com/crystal-lang/crystal/blob/master/src/comp... Seems pretty clear to me.
(2) Not entirely sure what you mean since that is such a broad case, but as stated, Crystal stdlib is mostly safe.