Big Pistons Forever
FBO and UERF loads should have been part of the load cases so will have been covered in the static analysis of the pylon attachments and wing structure (my experience is with stress analysis of A wings and this is true of the aircraft I have worked on so feel confident it would have been the case for B wings). In my experience we mix the engine failure loads in with the other groupings so the design is covered for some fairly nasty concurrent situations.
The FBO loads are likely to produce the peak engine failure loads as alluded to elsewhere in this thread simply because of the size of blades. This does not appear to be an FBO event so the peak design loads were likely not reached.
The amount of movement isn't cause for concern and the attachments can stand that kind of cycling for the time taken to get back to the ground. It doesn't look like there is any damage to the pylon structure in the video and the vibration to me looks to be consistent and well damped suggesting that all wing and engine attachments are still operating as normal. I would say the risk of losing the engine was minimal in this specific situation. The engine probably moves that much in strong winds anyway - but you don't notice it because it's an oscillation or 2 (for example there's a video of an A380 landing in Manchester with all 4 engines moving around and pointing in different directions during a winter storm/gales).