FWIW, the latest analysis I've seen seems to suggest that biomethane is probably the way to go for last-resort long term backup. Hydrogen is just too fickle.
What does it mean for a gas to be 'fickle'? Hydrogen gets stored long term all the time. It's made in refineries and used in chemical plants all over the world every day. It's expensive to store, but it's certainly doable with the right metallurgy. If your goal is reducing carbon emission as much as possible, it's way better than methane, regardless of source.
Hydrogen is a storage medium. It's fundamentally different than natural gas. Think of it more like a more complex, expensive, and inefficient battery; with the only advantage being that you can store more energy per unit volume at large scales.
The very small molecule makes it much harder to store than methane, and unlike anything else on the planet other than Helium, lost hydrogen is lost forever.
Methane is basically just hydrogen gas, but with one carbon for every 4 hydrogens. This means you can do stuff like liquefy it at higher temperatures and lower pressures, and it doesn't diffuse out of materials or otherwise destroy the materials (or seals) it's stored by.
>with the only advantage being that you can store more energy per unit volume at large scales.
In a vacuum, maybe; in real life, absolutely not. Hydrogen that is stabilized by a sufficient number of associated carbon atoms can be stored in plastic containers at standard temperature and pressure; naturally, we use the fuel that has just enough carbons that, while nominally a liquid, but would really rather be a vapor.
Because that stabilizer is part of that reaction, the best one from a CO2 perspective is the one that has as little of that stabilizer as possible. Which is what methane is.
Studies have shown that hybrids have fewer issues than ICE or BEV, though for different reasons. BEVs have new technology issues, and ICE has maintenance issues. Having a mostly understood ICE car that rarely uses the ICE and has the most reliable (battery+motor) part of the BEV makes a better solution short term.
A non-carbon containing fuel that is almost entirely created by burning fossil fuels is effectively the same (and often worse) than just using the fossil fuels directly.
Biodiesel or biomethane looks at first glance to escape this, but it has serious other costs.
Is almost entirely fossil-fuel created doesn't mean it has to be almost entirely fossil-fuel created. You know, just like observing that cars are almost entirely burning fossil fuels doesn't mean cars must burn fossil fuels.
Well, other than destroying the planet, but hey, let's just hide those costs.