Thanks for following up Katie. Today, you can share the markup directly with another user of Explorer (through an email, message, or AirDrop) and the recipient can use it in Explorer. Instead of sharing through ArcGIS Online (or perhaps in addition to) you could have workers share their markup directly to another device or other user. If you use an email, you could have someone make sure that message is forwarded to the users who need it before the next round of investigations.
This does also make the markup layer something that can be edited: markup shared in this way works on the recipients' device the same way their own markup layers work. You can't take notes in the same markup layer, as the edits are in your local layer and have to be reshared as a new layer, but I think it might still be useful for you.
I know it isn't ideal, as you have to know who to send it to, but perhaps it would be a way to operate until the next version of Explorer is out and supports sharing markup through ArcGIS Online, adding it to a map, and consuming it in the mobile app.
As for the differences in editing markup in ArcGIS Online and in Explorer, editing markup outside Explorer is something we don't support at this time.
Esri Education Team