Hope one can open workspace(s) without closing the current windows:
Add such a checkbox option in Start window, and in Customize:Workspace.
Actually, you can select a workspace while pressing CTRL key, and the current windows will not close.
Hope one can open multiple files/workspaces at a time:
You can use keyboard (SHIFT + DOWN/UP), or CTRL + click to select multiple items.
When Start window is opened (automatically or manually) in an empty tab, the text cursor will disappear, even when Start window is closed.
If you press the DOWN key to open the Start Window, the Restore Previous Workspace button will have the focus initially (no text cursor appears), but pressing the UP key will bring the focus back to the editor (the text cursor will appear). Otherwise, the text cursor should appear initially. If you believe there is a bug, please let me know how to reproduce the issue.