1. I don't think so.
2. Currently, no.
3. That script is for Apache mainly, you will need to remove all Apache part and update it to OLS. Or, run the script and make sure everything works, then replace Apache by OLS. with manually configuration.
Currently, no plan, but you could build it on FreeBSD.
I just tried to launch a FreeBSD v13 from Vultr, download the OLS source code, and build it with the build.sh script, and it seems to be working fine.
Note, the first time of build.sh will usually be failed, check the output and find the failed part, and go to the third-party/script/ and rebuild those packages. Then, go back to the openlitespeed folder and run the build.sh again.
If everything works, run the ./install.sh from the openlitespeed folder to install it.
Some 3rd package is also required, e.g. cmake go git pkgconf ..etc
litespeed is running with PID 17949.