Not answering your question either, but there are definitely issues running OWB 2.1.1 overall eg like
CLRT-74152: OWB 2.1.1 ETC and Actual Hours Not Showing on Resource Summary
which makes it totally useless.
In order to determine if you could run OWB 2.1.1 you should see the releease notes and the tech docs for issues fixed. The list of v13.x open defect does not list al and doesnot seem to be updated.
As far as running mixed versions CA has always forbidden that. However, as long as new version means new bugs users have tried to run mixed versions and it has been possible to run OWB from an earlier major version with Clarity. Though that has been in tests only. Like in this case I have tried to run OWB 2.1.0 and it seems to run OK.
Tried to look trhoug the release notes or v.13.3 and did not find the place where it explicitly forbids using OWB 2.1.0 Is it there or is just recommendation for utilizing the new and enhanced features? That has been the case earlier.
Martti K