When I build from kanjitalk755/macemu, I also get a fat binary that contains both 32-bit and 64-bit architectures:

But here it will run in 32-bit mode only. Activity Monitor then shows it to run as 32-bit in the "Kind" column. When I deselect "Open in 32-bit mode" in Finder Info for the application to force it to run in 64-bit mode, SheepShaver quits on launch like before with other builds since OSX 10.10.

Where do you see "Code Type: X86-64" in Activity Monitor?

Wait - I see that I built it for 64-bit architecture only, as shown here:

https://imgur.com/a/CjfvGbD

This also shows what Activity Monitor says:

Apologies for taking so long to find these details. It's been six months since I tried this.

I built with those settings. The result is a 64-bit only application:
The application quits on launch.

Note that lipo -info reports your SheepShaver executable to be fat, both 32-bit and 64-bit:

Are you sure that the SheepShaver you use is indeed running as 64-bit?

PM sent with (I hope) something you can try.

EDIT: About your last question, that first lipo report dates back to a different build from the one I made this morning. About the one I made this morning (and PM'ed you about), lipo reports:

And Activity Monitor reports:

I have fixed clipboard-exchange issue on 64-bit build.

Also, I have changed to use same clipboard routine (clip_macosx64.mm) for both 32-bit and 64-bit build.

By doing this, styled text issue may be fixed on 32-bit build.

https://github.com/kanjitalk755/macemu

Thank you! It builds and works.