a standalone executable (no installation required) that was a .map file and viewer together.
That's an appealing idea, but as you point out, there are significant security issues when launching an unknown .exe in Windows.
You can get close to the above right now. For example, you can distribute a .zip file that in the zip file has an unzipped portable installation of Viewer, plus the .map file you want to open. To make the .zip file smaller, you can just include either only the bin64 (for 64 bit operation) or bin (for 32 bit operation) parts of the installation. Hand out the .zip file to your recipients, and then they simply unzip and double-click on manifold.exe to launch and then open the .map file.
Or, you could just put the installation and .map file on a USB drive, and tell people to Run the USB drive (you can set what file is to be executed when a USB drive is autorun).
You could also provide a batch file that launched Manifold with the desired .map file, which is an option when launching Manifold in a command line.
Another way to simplify the above is if when Manifold launched it checked to see if there was a "startup.map" file in some standard part, like a data folder or the shared folder, of the installation hierarchy. In that case, launching Manifold would automatically launch with that startup.map. You could then package Viewer with the desired .map, and launching Viewer would launch with that .map.
I've sent in the idea for a startup.map as a suggestion.
There are other possibilities for being able to distribute an .exe that when double-clicked automatically launches Viewer with a given .map, but they are more complex and using .exe to distribute data is a security risk.
But if you're willing to take on such risks, Manifold could, for example, have an option to save a .map as an auto-installing .exe file. The .exe would consist of the data payload, plus a tiny executable that would a) first check if Viewer or 9 were already installed, and if not, b) download a Viewer portable installation, install it, and launch it with the data payload. In that case the size of the .exe would not be much bigger than the actual .map payload within, and you'd have the benefit that when users launched it they'd be working with a recent Viewer build.
But it's not clear what the gain is in that beyond simply providing a Viewer installation, and I doubt Manifold would be eager to introduce yet another risky use of .exe files for data distribution. Viewer isn't hard to install, and once installed you get an association with .map files, so users can thereafter simply double-click a .map file to view it. Viewer installation packages can be confirmed by their SHA to know the .exe is legitimate.