Regarding engines, I think there are three available choices for the Y-50:
1. High-bypass version of the WS-15 engine;
2. Enlarged version of the WS-20 engine; or
3. CJ-2000 engine adopted from the civilian version.
For #3 would be rather direct. For instance, the C-5M are using the same CF-6 engines as the 767, A300, A310 and MD-11. So I don't really see any problem with using CJ-2000 on both the C929 and Y-50 (depending which one is ready first).
For #1, WS-20 is a close-to-ready product, so it would be easier to start deviving a larger version from it. For #2, meanwhile, we might have to wait for quite some time, since the initial low-bypass version of WS-15 is still undergoing testing.
Actually, for China's case, the CJ-2000 might be the least favorable choice out of the three. CJ-2000, I believe, is primarily for the C-929, which is a commercial product. You don't want to give the US an excuse to ban your product, based on the allegation of "military application". In fact, as I understood, based on a few of Shilao's podcast, Chinese leadership is reluctant to showcase new satellite guided weapons on major news like the CCTV7, precisely because it will complicate international community's perception of the Beidou Navigation system.
From the same logic, I think your listed option #1 and #2 are perhaps better choices, in the eyes of Chinese decision makers.