There is a marker in the flywheel that the sensor picks up to determine tdc but as you say this is a 4 stroke engine which means that the crank rotates @ a different speed to the camshaft. Also the marker does not appear to be tdc on cly no. 1. With so many modern engines ther are no timing markings other than to time the camshafts & this is done when all the pistons are equidistant from the cylinder head as to allow the valves to open when positioning the camshafts. To determine tdc on no.1 i have viewed the camshaft lobe through the oil filler in the cam cover. The document you reference to is for the older engine, mine only has the one sensor, interestingly the newer vvc engine reverted to two!
If it does not remember where the crank is when stopped, then how come it fires before it does one revolution before it sees the tdc marker? Flywheel has not been touched nor camshafts, just complete engine out of one to another. I can see the gap in the flywheel, it is in the same position as other engine, I scoped the senor & it sees the gap. Swapped out sensor, swapped out coil pack, checked wiring, no faults in ecu. Swapped ecu even. Will check ecu to coil pulses next.
As Graham says Brian and I've said before, the ECU has no memory about where the flywheel was or stops, it simply starts again from scratch each time you attempt to start the engine. This is why the start is quicker when you have a cam sensor too.
Now if it is firing the mixture even before it has 'seen' the TDC sensor mark, that suggests a fault. Bear in mind though that if cranking speed is 240 rpm which is quite likely, then the flywheel will rotate 4 times in a second, so it will only take a max. of 0.25 of a second to start firing, and since the timing mark may be closer it could start firing even quicker.
Since you have a scope, you really need to scope the feeds to the coils and the firing pulses if you can. Injectors are not so critical since they don't need to be timed and very often are not. Some engines actually open multiple injectors at one time, because as they are injecting into the intake manifold, some may be injecting in at just the right time to be drawn in to a particular cylinder whilst others may be spraying in whilst the valves are closed but it will 'hang around' and get drawn in when they open which will only be fractions of a second later. You can work out the timing of opening and closing if you wish but at even 1000 rpm you are talking in milliseconds.
If it really is firing before the TDC has picked up and 'knows' the position/timing, then that might indicate you have a coil or coils firing randomly, maybe because of a short.
However, you have to go back to the fact that it worked before you swapped it over, and immediately after it won't start. So logic says it has something to do with the swap. Something that has been done, or not done, or some fault that has been introduced, such as a wire cut and shorted by the engine as it was lifted in or out... It is rare and probably very high odds that something unrelated has happened at the very same time.
Regarding the backfire of the Easystart in the inlet, be careful (in both senses!) not only of injury but in misinterpretation. By holding the butterfly open and using Easystart you alter the mixture ratio and because of valve overlap it is always likely that some ignition flame will come back in the intake.
P.S. Isn't this the engine with three coil packs at the 'front' on one bank of the V6 with H.T. leads to the rear bank? If it is, then you could at least get a connection in to the H.T. feeds to the rear plugs.
Roy