Graviman-
The first thing Garrett did with their CCE project was establish a baseline using the Nomad. Then they conducted numerous trade studies to determine what approach was optimum for cylinder scavenging, compressor/turbine arrangement, balance of work between the piston and turbo machinery, etc. Garret determined a uniflow (scavenge ports in the cylinder walls and poppet exhaust valves in the head) 2-cycle CI diesel core with an effective compression ratio around 8:1, combined with a single stage turbo compressor for scavenging/boosting, and a free turbine stage connected to the crankshaft by a gear set for power recovery, was optimum. The Nomad used different engine and turbomachinery arrangements than this.
A monoblock construction like Steyr's is an excellent choice for a high-pressure diesel, since it addresses many of the structural and heat transfer problems these engines face. There have also been significant recent improvements in piston rings, cylinder liner coatings, journal bearing materials, and lightweight forged steel pistons that would provide adequate service life when operating at peak cycle pressures well above 200bar. This may not have been possible just 10 or 15 years ago. But as you noted, the way the CCE achieves high efficiency is thru high cycle pressure ratios, so these developments are important.
Lastly, a 4-cycle turbo diesel would have lower thermal loading on the engine components than a 2-cycle compound turbo diesel, so it would likely have better durability. But the 4-cycle would also be heavier, and this is a significant concern for a rotorcraft application.