Wrong. Luci-ssl used to default to mbedssl, and now defaults to wolfssl (in master).
No such plans.
Openssl is intentional choice, as my build includes packages that only support SSL via openssl. (I removed vsftpd from the build but it was also such package).
LuCI itself has no knowledge of the underlying SSL library. Library is accessed via ustreamssl wrapper, that provides compatibility with supported library.
When wolfssl was due to WPA3 added to OpenWrt defaults to a few months ago, Luci-ssl was left with mbedssl, which caused casual users to have both in firmware. That's why I wanted to make sure that wolfssl is used also by LuCI. (Note that my own build uses only openssl)
No idea. I have never used it.