As mentioned in the comments, there is an inconsistency in com_users which requires to create the plugin in system group for both onContentPrepareForm and onContentPrepareData events to work.
Change group attribute value in the manifest file to system:
<extension type="plugin" group="system">
Change plugin class name to follow Plg&...
Do not create any login menu. This way it redirects automatically to the last page requested and attempts to login. For the same page, then use a guest page to a restricted page or you can use a login redirect to the same page.
Short answer is yes.
From the server side, the session cookie will be processed by the session handler if the joomla application is loaded.
There are ...