It does not work in my benefit so I'd refrain to give ideas to Ceolag.
But I think Fiona and her past tied to Elves and she does not like Elves. So, I feel like backstabbing Kaija in the future in favour of Elves will have huge negative effects on Fiona. If I am right and if we have an option to betray MC's family, Kaija and Templars that would lock us on Medb path and cannot pursue Fiona or any other LI other than Medb.
Why do I think that? On my last play through, I went for Fiona and Kaija love path to see if there is anything changed. I did not see any particular change but I felt like even after MC talked about what happened to him, Fiona was sympathetic towards Kaija and Templars. There is nothing I can pinpoint tbh and it's just a gut feeling but still thinking about being nice to Kaija so I can pursue Fiona makes my skin crawl.
Please, don't get any ideas from this Ceolag.