- I used the in-app sharing method and an email was sent to my coworkers. when they clicked the link they were redirected to OneDrive/SharePoint. When they clicked the document they were taken to the online view and even with "edit in browser" no other
people were visible in editing the document.
I was able to to turn on sharing under Review>Share Workbook (Legacy)>"Allow changes by more than one user at the same time" even though this error message popped up:

When workbook sharing was turned on, i was not able to open the document in the browser view, and was prompted to open it directly in excel instead. Sorry it's in Norwegian, but it says:

We're sorry. We cannot open the workbook in your web browser,
because it utilizes these functions which are not supported:
Do you want to open the file in Excel instead?
Learn more
| Open in Excel | | Cancel |
I proceeded to go back in Excel desktop and turn off the sharing (Review>Share Workbook (Legacy)>"Allow changes by more than one user at the same time") and when i tried to open it again from SharePoint, i was
able to view and edit in browser.
Regarding the original error message you requested from point 3. here it is:

PS: i also have added a link to the folder and document in my windows file explorer pane (left) for easy access. But i doubt this has something to do with the sharing issue.