I'll check on whether the coil is the right way round or not, but what is strange is that after that first start of the season and , say, a five minute run, it will start all through the summer on the 6V battery. Also I would say that I have checked the spark whilst on that first cranking with a 6V and it always appears strong. But I am only too aware that there are sparks and good sparks!!!
I was talking with Lyn Jones (who you have met) recently. He says that when he has a magneto that has "gone off" he takes the points out and rubs them up on what we call a whetstone ( a flat very fine carborundum block which we commonly use for sharpening wood chisels) and he says that usually does the trick.
I wonder have you ever come across the gadget that was used in old magneto repair shops for testing the strength of sparks. Apparently they were something like a glass globe with a spark plug and a connection where you could increase the pressure in the globe to simulate "under compression" conditions in the cylinder. A spark that will fire in the open will not necessarily fire under compression. I've searched for one of these gadgets at auctions but never come across one. They must be good - taking the guesswork out of assessing spark quality,