Old Engineer
You ask a couple of interesting question regarding ETOPS and engine certification. First ETOPS: ETOPS is granted after an aircraft/engine combination has gone through the complete aircraft type certification process. For the B-787, this process is not yet been completed. Therefore neither the RR Trent 1000 or the GE GEnx have received an ETOPS rating at this point. As step two of the process, a rating is granted on an operator by operator basis which requires crews and other personnel to be trained in ETOPS operations. And as you probably know, there are different levels of ETOPS granted, 120 minutes 180 minutes, etc.
Engine Certification is given as a result of demonstrating the capability of meeting all of the extensive certification requirements. The actual certificate may list only one engine model or multiple engine models of the same engine. If multiple engines are listed, it would mean that the base model went through the complete process and subsequent models (higher thrust) went through a shortened certification process to assure selected critical criteria conform to requirements. This would be a concurrent engine testing/certification program. I would suspect this is what RR did as they advertise the Trent 900 is certified at various thrust levels to 80,000 lbs. of thrust.
The GP7200 engine was certified by the US FAA and EASA. I suspect that RR didn't ask for a US certification as there is no current US aircraft application at the moment.
Turbine D