There is another aspect specifically of gas hybrids they typically use Atkinson cycle engines to increase efficiency but have reduced torque. This makes it ideal for a hybrid setup as the electric motor can make up for the lack of torque off the line.
Notice there are basically no diesel hybrids, this is because a diesel engine is more efficient than a Atkinson cycle gas and has even better torque than a Otto gas but they weigh more and cost more, so it makes little sense to add more weight and cost to a diesel drivetrain that is already very efficient and has plenty of torque. City buses are the exception due to the extreme stop and go low speed duty cycle that can really take advantage of a hybrid drivetrain.
Only the early-model Toyota Prius used Atkinson cycle engines, up until 2003 or so. You'd be hard pressed to find a modern hybrid using Atkinson cycle. Any modern Prius uses conventional engines with variable valve timing.
Most still seem to refer to these engines as Atkinson cycle even Toyota themselves: "The Prius retains its 1.8-liter VVT-i equipped Atkinson cycle petrol engine (2ZR-FXE)"
Kia Niro?
https://futuresuvs.com/2021-kia-niro/
"The centerpiece of the 2021 KIA Niro PHEV is a 1.6-liter Atkinson cycle four-cylinder petrol unit."
Thx for the nice explanations.
About diesel engines I read long time back that it's as well because they need to be "hot" in order to be able to filter their exhaust fumes (don't know how to say this correctly - the thing that is done by the catalyzer in the exhaust pipe) => as a hybrid engine is meant to be switched off quite often, the catalyzer of a diesel engine would have to be kept very hot by the battery, reducing therefore range/effectiveness/etc... of the hybrid system.
There are a few diesel hybrid prototypes, if memory serves me there was a two stroke diesel hybrid motorcycle called the eCycle that got something like 160mpg.
There's still an advantage to having a hybrid system onboard even at highway speeds. The continuously-variable ratio of the hybrid transmission allows increased freedom of the ECU to select {instantaneous gasoline engine RPM, instantaneous gasoline engine torque}, which permits operation in more favorable parts of the BSFC map more often.
Furthermore, the battery/motor-generators allow the ECU to temporarily run the gasoline engine at lower or higher output (without loss of traction power and without waste) than the power required at the wheels, further expanding the area of the BSFC map that the gasoline engine can operate at.
It is true that mechanical-electrical-mechanical conversion is more lossy than an all-mechanical system, but topologies such as the Toyota hybrid system allow for significant power transfer without electrical conversion; and spending significantly more time in more efficient regions of the BSFC map makes up for the conversion losses incurred.
Also, hybrids tend to have electrically powered coolant pumps and electrically powered air conditioning compressors, allowing for more aggressive fuel cut off (on a downhill section of the highway, say).
None of the other things you mention have more than negligible effect on the highway at cruise speeds where the combustion engine will be providing all the power to run all systems either directly or indirectly at typically ideal rpm most of the time. Also electrical cooling pumps and power steering etc again are not tied to hybrids and many non hybrids are going that route. A/C again does not need a hybrid but would exceed the capability of a standard alternator and direct mechanical compressor drive with a clutch is not that inefficient, again that would mostly be for stop and go urban driving at steady highway speed it probably more efficient to run mechanically off the engine.
A small diesel car and Atkinson hybrid will have similar highway mileage mainly due to the combustion efficiency of the cycles not the hybrid systems. However the hybrid systems allow a Atkinson gasoline engine to have acceptable performance in the city while the diesel doesn't need it. An Otto gas car will be behind but the cost saving can buy a lot of gasoline.