Shouldn't this be possible already?
to open a workspace(possible)
do things there(possible)
save this single window of this single workspace
If you uncheck include all workspaces and check save only tabs in current window. That should work.
close it and free all memory of it
If you only close the window, workspace stays, as it should. If you remove workspace it should be gone. There is option if you right click the workspace name in the drop-down.
to be able to see tabs info (urls, titles) of this workspace without opening
You can in the sessions panel when you click edit.
to be able to search urls or titles of this workspace without opening
You can in the sessions panel when you click edit. There is search bar near the top.
to be able to open it again as a single window with a single workspace as it was saved without affecting of other already opened workspaces (do things there, save it, and close it until next time).
You most likely can. If opening session does not replace the current one. You would open new workspace and restore session to that workspace.
I may be missing something since I did not test this. So if I am wrong please correct me.