Interesting. I think I see why this is happening, and can fix it for the next 25.0.2 hotfix. It looks like only New Projects for cargo have this problem, it's not using the value in the def-cargo-exe-path variable in one place. So even if you set that variable, it wouldn't prevent that error from popping up.
Until a hotfix is published, the only workaround is to add the directory your cargo executable is in to the PATH environment variable, and restart SlickEdit. The easiest way to do that is to start SlickEdit from a terminal like this, replacing "/path/to/cargo" with the directory cargo is in on your system:
PATH=$PATH:/path/to/cargo /Applications/SlickEditPro2020.app/Contents/MacOS/vs +new &
You could also modify the PATH in your .bashrc/.zshrc so you don't need to set it in the command line, but you'd still need to start SlickEdit from a terminal that has the updated PATH.
For the problem with the lockup, I'm not sure what that could be yet. When you open an existing toml, it shouldn't be doing anything I would have thought would lock up. If you can still reproduce this, wait till it seems to be locked, and then press and hold Ctrl-Alt-Shift-S for a few seconds. This should break the Slick-C interpreter, and give you a stack trace. Post the stack trace if you get one.