I have not followed this thread very closely, so I don't know exactly what your up to.
I think your trying to use a second transformer to tell the AC input a new voltage that is not the same as the voltage from the main transformer... in order to trick into thinking it is doing something it is not actually doing.
1. Your little 50watt or whatever transformer.... it would be nice to be in phase with the main transformer. It is not strictly necessary as it should be floating above the main transformers N output.... so..
2. The little transformer goes to N and L on the control card. This provides control voltage to control card
3. The main transformer line ....goes to the L input, and then through the CT and out to the load. So the controller can see the current through CT.
4. The neutral lead from the main transformer does not go anywhere near the control card. It floats... or actually goes directly to load point on your power receptical. This way your little transformer is only used for control voltage, and can be as small as you like. It will see no load, even though it is connected to the CT, the N rail is going nowhere other than the N terminal on the board.
5. That leaves the shaping capacitor. It needs to stay where it is to shape the voltage transformer wave, but a new one needs to go across the main transformer output... or go across the output receptical, so it can shape the power coming from the main transformer...
This means the little transformer is only connected to the main one at the L terminal only.... so it sees no load, the CT and the main will, as the CT is in series with the main L output.
So the new tranny is basically isolated from the load because the N rails are not connected to each other or the load N. It provides the new voltage reference only. It is connected to the CT L terminal, and the N terminal, but only to get access to the voltage dividers that control the output voltage.. it has no other interaction.
I expect the new tranny is connected primary to the main output, and secondary to the control card... ie a 220:240 or whatever transformer. This way, your load can drop the output voltage, and the new tranny will give a lower voltage to the controller to do the compensation.
Seems like a hard way to control the output, but it is your choice. I expect messing with the divider will achieve the same results..... but your show.
The CT has only 2 turns of pretty heavy wire, have not seen it suffer in all my usage so far..... but a current bypass wire will help too if you are going for high power continuous.
Is that how you want to do it, or did I get it wrong.