Open Excel file url in excel (from source, not downloaded copy)

No it is not possible for security reasons. All newer browsers (and I guess even the newer implementations of IE), don't allow file access to the local system. Internet Explorer is the only browser that supports opening files from locations, that are seen and treated as local ones.

You can open files from a Webdav Server, which is probably, why you can access files from your SharePoint, so you could try mapping that provider of your network drive as a Webdav Server.

Source: AND Is there an Application URL Protocol for MS Word?