1. It's up to you if you use the 50% failure probability and a large factor of safety or the 2.3% failure probability and a small factor of safety. In either case you need a factor of safety. I like to use the 2.3% curve, but it is not commonly made available. The 50% curve is typically provided for most materials.
2. Yes, you have a mean stress and the ANSYS Fatigue Tool can include a Mean Stress Theory.
If Analysis Type is set to Stress Life, choose from None (default), Goodman, Soderberg, Gerber, ASME Elliptical, and Mean Stress Curves. The Goodman, Soderberg, Gerber, and ASME Elliptical options use static material properties along with S-N data to account for any mean stress while Mean Stress Curves use experimental fatigue data to account for mean stress. You can specify the default setting for this property using the Mechanical application Fatigue settings in the Options dialog box.
3. If the high and low loads do not create what is called Proportional Loading, then you can use Nonproportional Loading in the Fatigue Tool and have exactly two Stress Results to put into the tool.
The options of the Type property are described below. Their availability is based upon your analysis type.
(r=0), (r=-1), and options are all constant amplitude, proportional loading types and are graphically illustrated in the Worksheet.
The Maximum Data Points To Plot property in the Options category.
option enables you to import a file containing the data points. This option is a non-constant amplitude proportional loading type. This data is depicted in a graph on the Worksheet. You can specify the number of data points this graph will display using the
The Non-proportional Loading option is a non-proportional constant amplitude loading type for models that alternate between two different stress states (for example, between bending and torsional loading). Problems such as an alternating stress imposed on a static stress can be modeled with this feature. Non-proportional loading is only supported for Fatigue Tool objects in a Solution Combination where exactly two environments are selected.
You should read the entire ANSYS Help file on the Fatigue Tool.