It is generally easier to shape the molding, including any rabbets and grooves on the back side, while the molding is still in long sticks. If you have already cut the molding into shorter pieces, it should still be possible to set the saw to cut a groove into the pieces one at a time. You can cut the groove using a dado set or dado blade or by using the quill extension. Take a pass, nudge the quill ahead a blade width, take another pass and so on until the groove is the right width.
As far as cutting compound miters, the chart in PTWFE seems to be accurate, but there may be an easier way to do the job. You can build a simple miter box consisting of two parallel sides and a bottom, making the distance between the parallel sides whatever it takes to get the molding wedged between them at the correct tilt angle. Then set your miter gage at 45 degrees and run the molding in the miter box through the saw. I know this is not a thorough or even very good description of the process, but hopefully you get the general idea. There are lots of good videos on line that show how its done. The beauty of doing it this way is that you avoid tilting the table. The method of tilting the table and angling the miter gage works, but introduces complexities, like dealing with the cut-off on the uphill side of the saw and the difficulty of adjusting the table and the miter gage to fractions of degrees.
Just a thought, but you can also use a good old-fashioned miter box and back saw. Some might consider this heresy, but hey, its all woodworking. I recently cut a crown molding that was 4 inches wide, which meant that I could not make the cut completely through with the Shopsmith, so I cut the last half inch or so with a Stanley miter box. Worked great.
Hope some of this is helpful.
Mark 5 of various vintages, Mini with reversing motor, bs, dc3300, jointer, increaser, decreaser