We will soon know what happened as they will have access to the QAR.
If the L eng was giving concern then even if the R eng was on fire you would probably throttle it just in case the L eng suddenly stopped.
Re the routing the a/c would have been at about 6000ft when it starting turning back so a dead stick glide would only give you 12nm. Clearly they had some power as they did a reversal at that point, to lose height & buy time. Well handled but some concern about two problematical powerplants??