I strongly agree that comments are key to maintaining complex forms. I extensively use Google Sheets comments and didn't realize that Excel linked comments to the absolute location of a cell. How frustrating.
Did you know that columns with names not part of the XLSForm spec are ignored by the converter? A common practice I've seen and recommended is to add a notes column, typically formatted in some way to stand out. For example, https://docs.google.com/spreadsheets/d/1DwyeaVg6BfhUGx8foTOtQCQaGx1p5FXayW77gOC1STU/edit#gid=0 shows a notes column in gray italics. I've also seen form designers include multiple notes columns including things like logic notes or relevance notes.
We don't have many of these best practices formally documented and would very much welcome PRs to add some in.