I have no send option in drafts, are you sure you do not mean outbox?

Under "File>Send Unsent Messages", the "Send Unsent Messages" is greyed out and unselectable. I have numerous unsent Mail Merged Emails in the "Drafts" folder. How do I send them?

You finish writing them, ie you open them and click send. Unsent messages that can be sent using the send unsent massages are in the outbox. If there was something in that folder to send, then the option would not be greyed.

I have 101 messages in the Drafts folder. Opening each one individually to send them defeats the purpose of mass Mail Merging. Is there a way to move those 101 messages to the outbox?

The 'Drafts' folder only stores emails that you have either written but not chosen to 'Send' or 'Send Later' for any reason or partially complete emails that you wish to further edit.

You cannot simply send any email from any folder except the Outbox. Emails must be selected be be Sent Later if you want them to be able to be sent later en masse.

When you choose to send emails, you can choose to either send immediately or Send Later. Send Later emails are stored in 'Outbox' folder located under Local Folders. You can then right click on Outbox folder and choose to 'Send Unsent messages'. Then all unsent messages are sent providing you are in online mode.

I'm not sure how you do the mail merge, but if you use this addon: https://addons.mozilla.org/en-US/thunderbird/addon/mail-merge/

then I suggest you choose the 'Send Later' deliver mode instead of 'Save as a draft'.