1) My information regarding the WS-10's dry/wet thrust ratio is not reliable at all. I cannot find any current standing data for the dry thrust but remember years back when the WS-10 was still at some 120kN, that someone had claimed the dry thrust was 89kN whereas the AL-31F produced 74.5kN dry thrust to 122.6kN wet thrust. Along with the civilian origins of the Taihang, it gave me the impression that it had higher dry to wet thrust ratio than the AL-31 series.1) As far as I know the WS-10 family has a BPR of about 0.8 as opposed to ~0.6 in the AL-31F, meaning its reheated to dry thrust ratio is unlikely to be lower. In other words, unless the version now fitted to the J-20 has significantly higher afterburning thrust than whatever AL-31F variant was originally installed, the WS-10 is probably not going to have more dry thrust. This is possible of course, but would only really be plausible if the old engine was a lower-thrust earlier AL-31FN.
2) Well, apparently not so trivial that it could be done for the initial series production batch with AL-31Fs, despite the obvious RCS drawback.
2) As I recall, early J-20 prototypes used AL-31X with a different silver nozzle. I don't see any difficulty in changing out nozzles or fashioning serrated nozzles to replace the standard ones; all the controls are still the same. It's not even like adding TVC. What would make it difficult?