Hi @pverley,
Thanks for reaching out!
The issue you describe with sorting by relationship fields in the Loan Preparations subview corresponds with an existing issue on GitHub (issue-5326). This is a bug concerning the inability to sort by some related, read-only fields on interactions subviews, which are unfortunately the kind you rely on here. While this issue is not currently in the upcoming development timeline, I left a comment on GitHub to notify the team of your report.
I also commented on the discussion for multiple sort fields to relay your support for this enhancement!