Some structures, such as a guitar string or a drum skin, behave very different globally when there is a pretension than when there is not. Most bolted structures behave the same globally whether there is bolt pretension or not. It is only the local stresses in the parts around the bolt that change.
That is why you can often replace the expensive computation of bolt pretension and frictional contact with a simple beam connection or fixed joint to evaluate the global response. The detailed model is made by cutting out a small piece of the global model, one corner in your example. Ansys has methods to enable this type of modelling by transfering the displacements from the global model into the detailed model.
You still have to get the detailed model to converge, but the mesh can be different.
Thinking about your 20 cycles vs 20,000 cycles, you only need to do 1 cycle and look at the values of stress. If the stress is below yield, then the structure can survive the 20 cycles. To determine if there is a concern at 20,000 cycles, you need the SN fatigue data for the material and do a fatigue calculation to predict the life for the peak stress in that 1 cycle.