My Ford towbar was fitted by a dealer, but I think they didn't replace the bumper fixings correctly or missed some as the bumper does seem to flex a lot now. If I push on the lowest part behind the wheel openings the plastic seems to move together with the wheel arch liner. Maybe it was always like that, but maybe you can compare and confirm?
The polystyrene inserts fit over the metal cross member but as far as I remember do not touch the outer bumper so give no support.
I had mud flaps fitted by the dealer from new. These needed to be removed to remove the plastic bumper. The dealer had not replaced one screws at each side allowing the bumper to flex more at the bottom, possibly giving the same effect that you are seeing. Check that all the screws have been replaced. I think 2 screws are visible and one is hidden behind the wheel arch liner.
As long as all the screws have been replaced then you should have no problems.
Yes this is exactly the process I went through a month or so ago, only i didn't take photos.
Took me about 4 hours to fit ( I'm getting old), the innerbumper and bits are stored in my garage. this was my one big dislike, that I couldn't refit the inner bumper. Mine is a Bosal detachable the same as yours.
P.S took me a further 3 weeks (on and off) to do the electrics.
BruceKuga pioneered getting the power cable through the main grommet in the engine compartment to the rear boot. I did the same.
I've never felt the same about my Kuga after seeing how flimsy the "rear bumper" is, and only held on by 16 screws (two invisible screws as you say)