Webclients have been outdated since Java started making really dumb fucking decisions about their security settings and clients started supporting resizable settings.
Would require a lot of time and a lot of work for something that would likely be inferior to the normal client.
Could embed the .jar and allow it to run pretty easily I'm sure. Wouldn't have to necessarily build a webclient from the ground up but it's been years so maybe things are different.
Not necessarily since most webclients have the applet run in a fixed size, meaning resizable/fullscreen wouldn't be possible without making a fairly complex webclient from the ground up, as well as changing a lot with how the client works to allow it to load the cache even while still in a browser.