Sorry to read about your issues, but something doesn't add up here and I agree that you need another mechanic. Ask them what do they mean specifically by "internal damage" and have them explain how that results in not getting to "half throttle" (another strange use of terms). Did they run a compression or leakdown check on each cylinder to make that determination? Do you have a check engine light and if so, what codes are locked in? Can you at least drive it to another shop?
Lou
05 Travel Supreme Envoy