No the engine fluhsing does not ruin the engine. Cleanliness of diesel & Mobil oil is of the utmost importance. Damage to engine can occur if diesel & Mobil are not properly and completely...
Yes ... it will need to be torn down and rebuilt.
Anything other than the required fluids needed to run an engine (gas, oil, coolant) can seriously damage any engine.
Short answer is yes.
if bleach burns ur hand it'll destroy ur plasticky bits in the engine and ull be feallin very silly