Why one way but not the other - just that pouring power onto the grid requires cooperation but producing hydrogen doesn't, or is there something more fundamental at play?

The fundamental principle would be that enzymes have an comparative advantage when utilising and producing complex organic molecules as fuel for hydrogen production (such as glucose in wastewater) while inorganic catalysts are cheaper, more stable but limited to simple reactions

Thank you!

If the enzyme is produced by genetically altered microbes like bacteria or yeast, wouldn't that make it practically useful on some scale?

Yes, that drives cost down but even on the biggest scale, like insulin, enzymes are still orders of magnitude more expensive than inorganic catalysts (e.g. platinum for hydrogen fuel cells)

Not necessarily. Where do they get the energy to produce that enzyme?

Is it more efficient than having them convert sugars to alcohol and burning it?