"Does Xara Cloud+ deliver the same functionality?" - Not currently but likely in the future

"For design files created before this version, is the Website > Other > Include WebP images setting Off?" - This option is off for all files, new or old by default.

"What is the situation for design Templates? Is the WebP setting an XDA selection or a Template one?" - Template/File one.

"When the WebP setting is on, how do you override it for a given image? There is no Image > Web image type: WebP option." - You can't currently.

"When I use Placeholder > Placeholder > Replace with a graphic file, will a selected image also get a WebP surrogate?" No, Anything within placeholders is not changed, with the exception of widgets from the CC in which they will be exported as WebP also

"Why does File > Export WebP, without any object(s), export the page contents when Export area > Selection is chosen?" No reason, as you say it's standard behaviour.

"Does your coding penetrate Placeholder codes and replace items like CSS: background-image: url(x.jpg); and JS references to pngs and replace them with WebP surrogates?" Any code within placeholders (with the exception of widgets as mentioned above) is not touched. If you have a link to a JPG or other image in the placeholder, it'll stay as the code specifies