A to your Q3+Q1: You need to read on in 8.6:
In other words whilst you can continue to use the startvm command, they prefer you just to start the relevant front-end directly e.g. VBoxSDL or VBoxHeadless.Note: We recommend to start virtual machines directly by running the respective front-end, as you might otherwise miss important error and state information that VirtualBox may display on the console. This is especially important for front-ends other than VirtualBox, our graphical user interface, because those cannot display error messages in a popup window. See chapter 7.4.1, VBoxHeadless, the VRDP-only server, page 91 for more information.
VRDP allows you to connect to the VM from another machine but does not support the seamless mode.