1) No, there's no extraordinary synergy between them, just enough to barely function as a "formation," which, otherwise, would just be 2 fighter ganging up on one.
The Du monks' formation has substantially more synergy due to the fact that 1) they're already individually better than MRF and YTZ and 2) they've spent decades to meditate to the level where they can telepathically hear each others' thought and therefore, move as one. JinYong always said that the biggest flaw of a formation is that the participants cannot move as one and therefore, in some moves, the best fighter in the formation has to scale back to the common denominator. The Du monks are able to overcome this inherent flaw by their telepathic ability and therefore, can move as one. So obviously, their formation is much better than MRF+YTZ.
Another factor you have to consider is for formations to be at it best, each participant has to abandon himself and trust that his partners will have his back, kinda like the Spartan phalanx. MRF & YTZ obviously didn't do this at all. In fact, it was MRF who slyly redirected all of XF's attacks to YTZ, which goes against the selflessness that formations need to work.
tldr; Du monks' synergy >>>>> MRF+YTZ's.
2) Of course. Then every single Wuji's "choke" could be attributed to plot device as well and we wouldn't be having this debate.
In all, if we go with "plot device" then both ZWJ and XF are not chokers.
If we only go with real events that happen, then XF is a much worse choker than ZWJ.
Don't have it both ways. Just pick one.
You see, with this logic, all the times that Wuji supposedly "chokes" can be attributed to "plot device", too