1) With 1, 2 or 3 phases active, no bridge can be used.

You are right, Erik! (Damn...) With more than one active phase, voltage heavily increases at electrolytic capacitor!!! Not a good idea, indeed, to connect three EMI filter + rectifier modules in parallel to electrolytic capacitor!
Even with increasing voltage rating of electrolytic, still transistors, transformer and other circuitry will suffer from too high voltage....

2) I assume that the input draw of the DC/DC is very small and thus there should be no need for all the protective measures (none were present in vacuum tube radios of my youth which, but for the tubes, were pretty reliable).

Protection circuitry also helps preventing damage caused by overvoltage coming from mains, like bursts and surges. But may be you are right and supply current is so small, that filtering measures, which damps interference caused by DC-DC converter, may be omitted. But I'm sceptical...

3) I hate Relays, they generate EMI.

I like them, because they are very reliable, right contact 'chemistry' provided. EMI is highly reduced by use of varistors and capacitors.

Thanks again for focusing the mistake, so hoping nothing bad will result...

Kai