You only have a 9 hrs rest because 15hrs shift in a 24 hr period means you only got 9 hrs left. Even if you do 11 hrs it will only count as 9 > so reduced rest.
I always follow this rule to stay on top things:
If worked more than 13 hrs (13:01 hrs for example) I can only have a reduced rest because within in the 24 hour period I can’t have my full 11hrs rest, so I have to take a reduced daily rest (because in this 24 hrs only 10:59hrs would coun’t towards my daily rest).
I hope everyone understands it.
Anyway there r companies out there which won’t rescue the driver after 15 hrs because legally it’s not allowed. I also companies which don’t have soo much “red tape” and these rescue and pay their drivers till they back at the depot.