Hi Trev,
I can't believe that you are experiencing so many problems. You have had more setbacks in the short period that you have been modelling N-scale than I have had in 25 years.
I know you say that the Class 73 is a good runner generally but it sounds to me that one or more of its wheels are "out of gauge" and that's why it cannot track through the points properly. A loco of this size and wheel arrangement should be absolutely perfect on all types of points, that is assuming that your points aren't damaged.
The problem of power not reaching the sidings could be a number of things. Are your power feeders in the right position relative to the points? If they are then the problem is likely to be a poor contact between the point blade and the stock rail. You can test for this by a bit of gentle prodding with a small screwdriver with the power on. The electrical contact can usually be restored by cleaning between the blade and stock rail with a cottonbud soaked in alcohol (not Newcastle Brown
) Also slightly bending the tip of the blade (as previously) tends to give better electrical contact. I would be surprised if you are having contact problems because these only usually arise after plenty of use when the track starts to get dirty unless, of course, you have somehow managed to damage the points when installing them. Do the points still have a good positive spring action? There is also the possibility that the rail-joiners are not making a good contact. Again you can test the joiners by prodding the rail gap with a small screwdriver with the power on to see if your loco moves. If you assembled and dismantled the track several times before finally attaching it to your board the joiners might have become slack. They might need a gentle squeeze with some small pliers.
I can't come up with any suggestions for the problems you are having with points not isolating.
Assuming your electrical feeder connections are correct it is impossible for Peco Setrack points not to isolate. There is a physical gap of about 2mm to stop the current flowing!! If I remember correctly there are two wires beneath the point separated by an insulation sleeve but even if this insulation had failed it would result in a short circuit rather than stopping the points from isolating.
I assume you are using Peco points because some of the foreign makes (e.g. Minitrix) have small wire links that have to be removed to make them into self-isolating points.
I really hope that you can get to grips with these problems Trev because I think you will soon get bored with the hobby if you have to resort to a simple loop with no points.