If you haven't seen it, I'd also recommend looking at the excel1040[1] spreadsheet. It's been developed for about 20 years by a single author, AFAICT. It has numerous forms, schedules, and worksheets, and it makes aggressive use of formulas and locked cells to ensure that calculations are correct. The spreadsheets closely resemble the actual forms, though I don't think people are actually mailing them in as-is.

[1] https://sites.google.com/site/excel1040/