If I send a document to be signed can I edit it after? I will need to insert a date into the agreement after. Also, the signatory will need to have 2 witnesses verify as they sign, will they be able to update the document to fill in witness names?

Envelopes can be "corrected" but there are limitations. For example you could not replace or change the document if a Recipient has taken action. You cannot remove a Recipient or alter Recipient field data once they have completed actions. You can add a Recipient and Recipient fields for the new Recipient but it has to be done prior to envelope completion. If you need to add something to the document, just add yourself as a Recipient, assign yourself fields. You can also use the Specify or Update Recipients (Recipient Types) to add the Name and Email of unknown Recipients, but I would suggest reading up on those types called Manager Recipient Types. Here is a DocSign article...

https://support.docusign.com/en/guides/ndse-user-guide-manager-recipients