Yes, BE16 FP2 includes support for Exchange 2016 CU5 (and CU6). You can see the complete list of supported applications on the BE16 Software Compatibility List: https://www.veritas.com/support/en_US/article.000115689
Yes, you can upgrade straight to either Backup Exec 15 or Backup Exec 16. The installer will detect the existing version/data and install over that version, retaining the data. Bakup Exec 16, however, is not supported (and won't install) on a 32-bit ...
The SLF/license key is all you need, and that should enable the CASO feature. The bits should already be there to get the option installed, it just needs to be unlocked (with the SLF).