Unable to Open an Old OST File After Outlook Profile or Mailbox Changes

The best method would be to extract the data from the original mailbox. If its been a while and the mailbox is no longer available in soft deleted or inactive state. You may run a eDiscovery tenant wide with the person in participants, this will pull together any email that this person was around using the existing mailboxes as multiple sources to consolidate almost a good picture of the original mailbox content without raising any privacy concerns.

Answers your questions, which is mostly not supported by MS:

• Is there any Microsoft-recommended method to access an orphaned OST file? - No
• What checks should be performed before trying to recover data from the OST? - Original PC with the profile intact, can be tried loading in Offline mode
• How do administrators usually handle old OST files when the original mailbox is no longer available? - OSTs are not meant to be access offline without the actual profiles intact, its just a cache for live\temp usage only.
• What is the best way to validate recovered mailbox data? - N/A. There isn't much to validate, if you don't have any reference to match with, just check the items for To and From