I think thats pretty clear the receiver battery must only supply power to the radio receiver.
By plugging it into the ESC you are supply power to the esc which is then supplying power to the receiver. There is no way for a scruitneer to be able to tell what the esc is doing with the power.

In my opinion, if it were legal to do that regulatory bodies would need to get sample escs, reverse engineer them and then verify they are legal and put them on a list. Otherwise anyone could rock up with their own esc and you would never know what is happening.

As for the tekin that is something i have said around the tracks. I know functionality is different when the esc is off. It doesnt have a voltage cut off when it is off (which to me would theoretically mean that if the esc is off it is not legal, but then again voltage cut offs dont seem to do a lot in 12th scale). Tekin would have to comment on what other functionality does, doesnt or kind of works when the esc is off.