As you mentioned, each of the settings in the FFF window includes a tooltip that can be viewed by hovering your mouse over that setting. A lot of these questions are answered with those tooltips, but I can try to add a bit more information here.
1) The "extrusion" refers to the plastic spaghetti coming out of your nozzle. The extrusion diameter is technically the cold measured diameter of this plastic extrusion after it has been extruded from the nozzle, but many times this is nearly identical to the nozzle diameter. You usually don't need to tweak this setting further than setting it equal to your nozzle tip size.
2) The auto extrusion width option is used to automatically calculate your extrusion width based on the choice of layer height, nozzle diameter, etc. This width is the width of the plastic after being squished into each individual layer. If you need to manually define this width, you can uncheck the automatic extrusion width checkbox and then enter a value in the box beside it. The automatic calculations are very good though and we rarely need to do this unless we are using very extreme layer heights.
3) The minimum infill length is used to avoid printing lots of very tiny infill segments. Many times these small infill segments are not critical to the part so they can be neglected to save printing time. It's usually safe to use a value between 5-10mm without adverse effects.
4) When you enable the generate support material option, you are just telling the program to include the G-Code to support the model. If you define manually define support for the model, but do not enable this option, it should warn you that you probably intended to have it enabled. The automatic support material is only generated if no manual supports have been defined. You can use them together if you want by combining the different option in the Support Generation dialog (for example click the "Apply based on overhang angle" button to begin with automatic support and then use the "Add new support structures" button to add some extra support in addition to the automatically placed regions).
5) There should be no difference in these options, but using the diaphragm for this is very unconventional. That is typically used to add spaced horizontal ribs into a part (usually only once every 50 or so layers). I would also advise against using 100% infill since this can create serious problems with warping and rarely produces good looking parts. An infill percentage around 60% will usually be solid enough for most purposes.
6) After you click the Prepare button, the program will evaluate all of the detail extrusion width calculations based on your FFF settings. If you open up your FFF settings after you Prepare the file, you can see the extrusion width that was used (it will be overwritten in the textbox).
Hope that helps!