Software :: Unable To Transfer Emails / OSX Mail Not Importing Old Inbox

Acquired new MacBook; attempts to import old OSX Mail from ailing G4 PowerBook only partially successful.

First attempt: took copy of "Mail" folder from "Library" folder, and copied it into MacBook's "Library" folder. Opened Mail, and my minor folders (4-5 mail folders with no more than 100 messages each) appeared OK; no sign of my inbox mail (1,300 messages), or sent mail (2,500 messages).

Second attempt: discarded "Mail" folder from above. Ran Apple Backup to restore Mail files. Selected over-write everything option. Same results as above.

Third attempt: tried to import the mail boxes using File: Import Mailboxes function. Each time I select the folders with the emails I want (my old Inbox and Sent mail), the import wizard says" No valid Mail messages found". However, if I just manually open those folders, and double-click on any of the mail files (all with .emix extensions), I can open each email.

Am currently pulling out hair trying to figure out what I'm doing wrong. Old PowerBook files were encrypted using File Vault. New MacBook was encrypted using FileVault after attempt 3 failed above, and then de-encrypted when that didn't work, and the above steps were repeated, to no avail.