On point A I say never stopped them with Boeing 737s. Having the Line in China as well as many components means they have control. Additionally we know they have been working on Indigenous major components such as engines to replace imports.
My understanding is that many of the major C919 subsystems are US origin.
The C919 and Y-9 have roughly the same payload and MTOW.
So why bother with the hassle of using the C919?
In comparison, the Y-20 is in a different weight class as it is 3x heavier than the C919 or Y-9.
Next China needs Tankers Quote unquote but specifically for use with rotary wing. It’s true China doesn’t have any in flight refueling Helicopters then again only one model they had was comparable the S70. Now with there own types emerging and expanded operations they may chose to expand the line.
In what scenarios would China need to refuel helicopters?
On the Chinese mainland or in the First Island Chain, it's better to use a tanker truck on land or a commercial ship with a helipad.
An airborne tanker is a very juicy target.
It’s true for the tanker mission Y20 would be better yet those same notions work for AEW platforms. The Y20 being quad engine means more fuel consumption over longer range vs the twin engine that gives you both that range and efficiency. As such I again point to the Comac line for possible in the longer term. The 919 for tanker/ASW/JSTAR/lower VIP, 929 as Tanker/VIP/Freight.
I do see the Y-20 being used as a future AWACs platform.
You need the additional speed and range to operate offshore in the waters past the 1st Island Chain.
But closer in, there isn't as much difference between a Y-20 AWACs and a KJ-500.
---
Jet Fuel is actually pretty cheap when you think about it, and not really an issue.
Say a Y-20 carries 100 tonnes @ $4 per kg. That works out as $40K of fuel in total