Given standard gauge trains transverse plenty of bridges which frequently have high-winds, that seems… dubious.

Yes, this is a popular theory but there's no evidence in the fossil record.

Is there evidence in any direction? This really sounds like the most plausible hypothesis, to me at least.

With strange, custom-construction-requiring stuff like this, I tend to wonder how much of it would be explained by "help self/a buddy get rich". It's practically guaranteeing that there's going to be a single company supporting it for all time, which is a potentially-extremely-lucrative business deal.

If that's the case, obfuscating the reasons for these decisions is a rational move.

It's a bit cynical to say this but running HSR on an incompatible network protects it against future politicians who may seek to sabotage it by merging networks.

This sounds similar to the reason why the Glasgow Subway runs on a 4' (1219mm) gauge: to stop it from being taken over by the numerous (at the time privately owned) railways around the city which were all standard gauge.

Marin was not a participating county, and that was established long before gauge would have been chosen.