Sorry Jeff, I missed your inquiry at the time. Hope you were able to resolve the problem.
Always remember Scott was a custom set builder, and the Scott Fifteen was constantly evolving. There are 5 known factory diagrams, so far. The early production were Wunderlich detector sets, also identified by the additional pair of red antenna posts. The diagrams in Riders are the last 2 versions using a 55 for 2nd detector. Even those 5 diagrams do not cover all the 7 (or maybe 8) variations known. Each of the IF stages can be considered identical except the last one. The final IF coil for the Wunderlich sets was center taped and the ends feed the 2 grids in the Wunderlich tube in a push-pull fashion. The first of the sets using the 55 detector set is not in Riders, and also used the center taped push-pull final IF coil, and the two ends connected to the 2 diode plates of the 55 tube. To further confuse things, when Wunderlich tubes became unavailable, some sets were converted to use the 55 tube.
For your 3rd IF problem, check coil continuity, replace the cap inside the IF can and the continuity of the coil below the can inside the chassis. Also, make sure the grid wire is not shorted to the braided shielding, that the tube is good and that the tube socket is sound. Either rebuild the metal bath tub caps under the IF section, or substitute modern ones of like values - depending on the model variation some of those bathtub caps have coils inside too..