Platform or a collection of special portable editions

In comparison to virtual registry layers like U3, Ceedo, and PowerToGo; is PortableApps suite actually a software that needs to be installed at the USB Flash prior to installing portabel applications?

no..
any PAFed portable app hosted here can be used after install whereever you put them.
ThePortableApps platform is just a menu helping you to organize your portable apps on a drive providing some extra gimmicks like backup, autorun, wallpaper switch etc..
The suite is a package containing the platform and the top download apps coming with one "installer"

And would the data files of portable apps retain their format, so they could be copied back to a HD and be used for an actual HD-installed application if needed?

yes and no

data file contains the local profile from %appdata%, eg Firefox, but you can`t simply put them back to local profile because PortableApps uses a sort of simplified profile folder structure versus the local %appdata%. You could restore single files within the profile easily because they have the same format but simply copying back the profile folder to %appdata% doesn´t work in every app