What are the chances of the RR Trent fleet being grounded? (G-YMMA to G-YMMO)
I think this happened before on the first B757s, without such a dramatic outcome. IIRC for a while there was a minimum N1 that had to be maintained on the approach to prevent the engines going into a sub-idle condition in the event of a particular failure (I can't remember what. I'm still trying to find the appropriate technical newsletter in my archive).
This permitted the aircraft not to be grounded while checks were made and the requirement was then lifted. This sounds quite similar.
The photos of the wreckage show the right engine blades intact suggesting it subsequently completely ran down before impact, while the left engine blades are torn off but with little damage to the casing suggesting it was still running at impact but couldn't be accelerated out of sub-idle.
As regards the co-pilot continuing to fly the aeroplane, I think this was a good decision by the captain, enabling the more experienced pilot to try and find the root cause of the problem. Unfortunately there was just insufficient time remaining at such a low altitude.