You are pretty much describing the end-game scenario I had in mind: the use cases where CO2 capture would be hard or impossible are mostly the same as the use c

You are pretty much describing the end-game scenario I had in mind: the use cases where CO2 capture would be hard or impossible are mostly the same as the use cases where direct use of H2 (without methanization or even further synfuel steps) is prohibitive. Mobile (mostly aviation) and decentralized (upgrading distribution networks for H2 capability down to individual kitchen gas stove seems almost as unlikely as upgrading the stoves to capture). A post-fossil energy economy that combines H2 (where feasible) with methane (where necessary) won't see much if any of the methane's carbon captured. In a post-fossil economy CO2 sufficiently concentrated for methanization could easily be a limiting factor. Photosynthesis is the only scalable way to concentrate carbon from an atmosphere where CO2 is measured in hundreds of PPM, so all the post-fossil methane has to come either directly from biological processes or from biomass burning capture + H2.