Should os.Getenv return an empty string or an error if the environment variable is not set? -- The answer to this depends on how unix-y you are, I suspect. We might disagree, but it's not wrong to say that you're going to do what bash does, which is, after all the fundamental place these are kept, and return an empty string. In particular, this is well documented in the API, regardless.

If I create an empty list, then marshal some uninspected text into it, and that marshal function fails but I don't check its error status, should my list be poisoned, or should it continue on as empty? -- The idiomatic answer to this is that you should check error status and remediate.

He would prefer that the program vomit when marshaling fails. Fine, but it was entirely his choice to code in this style, against go style guides. To then claim the compiler is 'unhelpful' later when he tries to read the list he declared is empty is blame shifting in my opinion.

I would guess he does not prefer in-line error checking and remediation as a pattern; that's totally fine, but it's annoying to read snark about it.