If you want to cover all the "dumb" devices like car stereos and old phones... you're stuck with MP3, I'm afraid.
Opus would give you better quality than HE-AAC, but it would probably make at least some users install new software.
So I'd go with HE-AAC. I don't know for sure which HE-AAC encoder is the best, I'll let someone else chime in on that. But I think Apple's (iTunes) encoder should be a safe choice, with something like 48-64 kbps VBR.
And I don't think HE-AAC v2 has any advantage over HE-AAC v1 for mono signals, but correct me if I'm wrong. When I try to encode a mono file with v2 in foobar2000 I get the exact same result as if I select v1.