My past experience tells me that once an engine has been operated,
it is considered a dangerous goods because of the fuel/oil involved
in the operation. How is the freight forwarder going to determine
when was the last time the engine was running?
If you can provide a copy of the regs you quote that state after a specific
time frame a used engine is no longer considered a dangerous good,
then you might have a case. But it is up to the airline receiving the
cargo whether they will accept that.