Rev high when changing gear up

It may be worth looking at the timing belt tensioner pulley bolt to see if it hasn't worked loose, the belt may be about 3/4" of slack, which could cause the belt to slap against the inside of the cover at low rpms. It can also slowly shave away the backing of the belt. There could be about half a cup of timing belt dust/shavings collected in the bottom of the lower timing belt cover.

Is it possible the clutch pedal is interfering with the throttle? Start the engine in neutral and allow it to idle. Press only the clutch pedal down and see if this increase the engine speed. If it does speed up the engine and slows back down when you release the clutch then somehow this operation is where your fault lies. If it doesn't alter the idle speed, then rev your engine and release the throttle to see if it returns to idle immediately. If it is slow to return to idle then your throttle cable could be sticking. This means when you change gear lifting off the throttle has little effect and depressing the clutch to change gear causes the engine to race as the throttle is still partially open.

do they revs drop off when taking pressure off accelerator pedal when out of gear? if only staying high during gear change it would suggest a worn clutch friction/pressure plate so the engine isn't slowing down when clutch is depressed .I'd also have the OBD codes read to see if there is a sensor/switch at fault , but sounds like the clutch to me

sounds like a failed clutch or broken transmission

It is possible he installed the incorrect gear lube in the transmission. It uses ATF no gear lube.

Yes it is the clutch slipping, in high gear the greatest load is put on the clutch so that is when they slip the most.

It needs rebuilding. The upper gear(s) are not locking in, so your likely wearing out your clutch when you drive fast. Get it rebuilt.

is it fly by wire throttle control ?? or a conventional cable

The grinding sound may be a throw out bearing or the pilot bearing. The sticking throttle problem sounds like a linkage or cable binding up from possibly damage due to the impact from the accident.

it is just one of those cars that drive like that its different but you need to change your driving style to suit