For 1 & 2, the issue wasn't that the author was looking at the wrong logs/coredumps. It's that coredumps from inside containers typically don't match the symbols available outside the container - you either have to run gdb inside a matching container, or rebuild the contents of the container in the host environment (as they did here).

3. There's nothing wrong with the level of abstraction here. If you have a crash that occurs on ARM but not on amd64, the differences in how those architectures operate is a very reasonable initial assumption.

4. The value in x20 is the same value in the local variable in question. Even though there may not be a general one-to-one mapping between variables and registers, at this particular instant in time that variable does correspond to this register.

5 is irrelevant, as the article isn't discussing forking. It's discussing the (somewhat questionable) practice of a program using getenv/setenv as mutable state.

For 6, the article doesn't say that env stores its own array length. It says that setenv called something like free() on the old env array, and free() overwrote env with the length of the memory allocation (which is a quite reasonable way for malloc to do book keeping).