Thank for the follow ups and I am glad that you solved the problem.
For your information, the FinalIK integration clips in Slate are pretty much just calling FinalIK methods (not any custom processing take place other than updating the IKSolver of FinalIK when it needs to). It is also better to use FinalIK clips in a track above Animator Track because without animation playing, the FinalIK solvers will not work as expected.
If you have any more questions (or issues), please let me know.
Thanks.