Via an XC ticket (we have several tickets currently, none are connected to this as it happens, but thought it worth a conclusion of this topic anyway FWIW) the answers are:
(Quote)
"resized images are regenerated when a new image is uploaded to a product" i.e this means not at any other time e.g. using a viewing option that calls for a resized image. No details of how a 'new image' is verified
but a manual re-generation does remove any doubt, which is fine.
(Edit) 'Progressive resized jpegs will currently not be produced from progressive jpeg originals via Generate Resized Images located within Look and Feel in XC5.2.* A Custom Modification can be provided if this is required' That's it in a nutshell.
We suggested this feature (standard like-for-like production) should become part of CORE XC 5.3.* i.e. progressive jpeg > resized progressive jpeg or interlaced png > resized interlaced png etc