Concerning Airbus approach:
There is a stress increase due to a dent. Depending on the ratio of skin thickness / dent depth there may occur a stress increase of up to 80 % for deep dents.
You will find in the A320 SRM (it seems you have access to it) in chapter 53-X1-11 pb.101 (X for the...
The Pylon structure is of course very sensitive. In this case I assume there is also a required fit of the pylon to the engine.
You should have a look in the ACD's for the minimum reserve factors of the pylon and the coresponding load cases. With this data you should be able to recalculate the...
The safety factor of 1.33 is for separable connections.
So there's no need to use it for the seat itself, but for example for the stud to attach it to the seat rail.
Of course this is roughly the way to do it in case of tensile loads. But if the structure (what kind of structure?) is loaded with compression it might be a bit different.
