No - the messages in the folders would not be available on the server unless the Outlook settings had previously asked for messages to be left on the server.
With regard to the large number of messages that must not be lost - I would not leave them in Outlook, but archive them saved onto an external storage device. Also, from a program operational point of view it is not recommended that you should create sub-folders from the Inbox, or any of the system folders. This can cause Outlook to crash, particularly when high volumes are involved. They should be set up as sub-folders of the Local Folders heading.