Repair Mortgage Amortization Excel

I have an existing mortgage-amortization workbook in Excel that needs a precise formula and logic overhaul. At the moment several figures are off: the total payment summaries aren’t matching reality and, more critically, the balances reported at different periods drift from what they should be. Because those totals feed directly into our Net Tangible Benefit calculation, that metric is now unreliable too.

Your mission is to trace the formula chain, isolate the logic errors, and rebuild whatever parts of the sheet (or any supporting VBA) are causing the mis-calcs. While you’re in there, I’d like the file streamlined so recalculation is quick even with large input changes.

Also, need to create "Effective APR & Payment" should the loan be paid off before 30yrs i.e. after 6 months, 1 year, 3 Years, and 8 years.

Deliverables
• Corrected formulas (or VBA) ensuring payment summaries and period balances reconcile accurately.
• A reliable, automatically updated Net Tangible Benefit section reflecting the fixed totals.
• Brief change log so I can follow the adjustments and maintain the sheet going forward.

Acceptance criteria
The workbook must reproduce industry-standard amortization schedules for a range of interest rates, terms, and extra-payment scenarios without rounding drift, and recalc time should remain under two seconds on a typical modern laptop.

If you’re confident in auditing complex Excel logic, let’s get this sheet back on track.