From the IRS website: "the IRS doesn't endorse or approve any particular software for IRS e-file." I have had calculations by tax software corrected by the IRS and Revenue Canada in the past. The IRS does not have the resources to test software with all possible scenarios.
Your explanation of the tax calculation is incorrect. A taxpayer with excluded income only pays a higher tax rate if the tax calculated on all income, using the preferential capital gains rate, exceeds the tax payable on the excluded income alone calculated at ordinary rates.
