iPhone photos are often HEIC, which some programs and upload forms can't open. Convert to JPG in seconds.
One file at a time, and the first run can be slow because the converter has to load. Very large files may take several seconds. Another option is to set your iPhone camera to Most Compatible.
Slightly, as with any JPG.
Yes, through any browser.
Yes. It runs in the browser on iPhone and Android, with no app to install.
Nowhere. Files are processed only in your browser, are never uploaded to a server, and are gone when you close the page.