To build 919 indigenous is technically not out of reach for the chinese aircraft industry. Why the 919 is using so many sub components from foreign companies is more economically driven than technically.
Comac need to get the FAA certificatian if it want to push the sale of 919. Without the FAA certification, 919 is even for chinese domestic airlines not very attractive. With sub components from foreign companies,who have much more expertise in producing this components and where their product are already used in other aircrafts, it is much easier, cheaper and most of all faster to get the FAA certifation
with sub components from notable foreign companies than using domestic one.
For the military use of 919 where no FAA certification is needed, they shoudn't have any problem using domestic parts. Using domestic parts, it will maybe´cost more and the quality might be as not good as foreigns at the beginning, but it is technical doable and it will help to develop the chinese aircraft industries for the future.