For whatever reason I cannot import other M9 .map files into an open M9 map project that I'm working on.
Only one 9 session at a time can connect read/write to a given .map file. Connecting read/write locks out other sessions. If you connect read-only (check the box when you open the .map), then a second 9 session can import that .map file, connect to it read-only also, link to it, etc.
If you want to run multi-user, having tables and drawings that multiple different 9 sessions can read/write, the way to do that is to store the tables/drawings in a multiuser database. Choose any of the free ones and it works great.
The work around is to import the Shape file I used to create the .map file I tried to bring in to may new project.
Not a good alternative: shapefiles kill data because they force storage of highly sophisticated, modern and rich data into brain-dead 1970's era data types.
There are far better alternatives to bring data from one .map file into another, including...
Copy and paste works if I select the drawing and table components in the project pane of the file want to import.
Copy/paste is a great way to copy data from one project and to put it into another.
Doing a Crtl-A of all the objects and a paste into the new project pane didn't work either.
More accurately, you can't Ctrl-A to Copy All because that would be a Copy of the System Data folder, which is unique to the project. You can Copy everything except the System Data folder, for example, hundreds of tables, drawings, images, labels, comments, queries, etc., and Paste those just fine into a new project.
By the way, if you want to use absolutely everything from one Manifold project in two sessions at the same time, why not just copy/paste the project in File Explorer and open both copies at the same time in two different Manifold sessions? Or, if you like link in one copy into some other project you're working with. If you have archival data that's often used in different projects, just keep that in some .map file that you link in read-only and you can use it in however many sessions you want.