Re: ero emissions?
"Third, yes I do understand that air / nitrogen / helium etc. is not the same thing as hydrogen and specific care has to be taken in the choice and engineering of the material used, but declaring that "problematic" sounds a bit loaded."
Air/nitrogen/helium at any pressure don't cause hydrogen embrittlement (very similiar to neutron embrittlement in the nuke industry). It's bad enough in the CNG industry and the hydrogen there is generally bonded to 4 carbon atoms already (If you've ever seen a CNG tank burst you'll know how dangerous that can be - cars generally get shredded when it happens)
"Problematic" is actually an understatement and the cyclic nature of the pressurisation along with low mass requirement for transportation add an order of magnitude of complexity to the problem. https://en.wikipedia.org/wiki/Hydrogen_embrittlement