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.