A MySQL server was initialized with separate UNDO tablespaces.

Yes. Shut down the mysqld process, enable the transportable_tablespace option, and move the UNDO directory to the SSD.

Yes. Shut down, copy the UNDO tablespaces to the new location, and change the innodb_undo_directory value in your my.cnf.

No. The UNDO tablespaces must remain on the same file system as the system tablespaces.

No. The sequential write pattern of the UNDO tablespaces is not supported on modern SSD block devices.