I built a Mac app that edits .mpp by patching bytes instead of converting. It's out. Here's the sample file, open it in real Project and tell me what broke.
I ran Microsoft's developer division for six years as a GM/VP, then three years at PwC leading the Microsoft practice. The whole time, the thing I heard constantly was some version of "I'm on a Mac and the client wants the .mpp back."
Every Mac scheduler that "supports" .mpp reads your file into its own model and writes a new one out. That's a conversion. Anything the model has no slot for — deadlines, task splits, subprojects, external links, task calendars, resource notes — is silently gone. Not corrupted. Just absent, and you find out on the call.
I built the other thing. It patches the specific bytes of the specific fields you changed and never touches the rest. Changing a percent complete moves 2 bytes.
I'd rather you test it than believe me. There's a sample .mpp on the site that the app wrote on a Mac — tasks, dependencies, resources, baselines, costs. Open it in real Project on Windows. If something's wrong, post a screenshot here and I'll fix it and reply with the test name.
What doesn't work, up front: password-protected files (Microsoft's encryption is undocumented, and that's the entire loss list), MPX, Project 98 through 2003, no Project Server, no SharePoint, no ODBC.
Free tier opens and edits any .mpp of any size with no row cap. 30 round-trip capabilities, each pinned to a named passing test.
Happy to answer anything about the file format. That part's more interesting than the app.