Hi again,
Ok, indeed the mystery is solved now.
When changing my scenario to multiple shipping methods offered, I also stumbled over the bug that the default one wasn't pre-selected, as reported also by others here in the forum. That caused nothing being displayed with "read only", obviously. And installing the latest and greatest as suggested by you fixed it all then. So, thanks for that.
Please allow me this final note:
When I said
I wouldn't know what "read only" would be good for if there were multiple usable shipping methods on offer
then because I interpreted "read only" strictly as that: options displayed (all, if multiple), just no way to select. Admittedly, I had not yet read the documentation on that particular... but who does that when things seem obvious...
Hence, I'd think changing the "Read only" label to something like "Only display default" could be easier to understand?
Or make it it the mentioned option instead: if enabled, in case of multiple usable methods it'll give the customer the choice to select, but if there's only one single method it displays it as "read only"?
Anyway, case closed for me. Thanks again for the fix.