The advantages I see of this process are: Ease of storage, re-use of the coal plants, ease of fuel generation.

Methane seems nice for the re-use of gas fired plants. I do think its gaseous nature has more downsides than upsides on the other two factors. There's also a gas-leak hazard with methane that doesn't exist here.

If the capital requirements are low enough, I could see a 40% efficient energy battery for smoothing out peaks being nicer than more complicated storage schemes.

Chemical batteries will likely eat the smoothing market.

For fuel that can be transported, methane is a winner. In addition to the distribution infrastructure, there's an awful lot of residential use, with lots of appliances that don't need replacement.