Dear Q7 owners - I am looking for some ideas on what might be the problem.

I set in motion a string of catastrophe...possibly...but hopefully not

On a 2007 Q7 4L bug engine 3.0 TDI with 147,000 miles on clock I successfully replaced an injector seal 6 months ago on cylinder 2.
Car worked perfectly and was notably quieter in engine bay.
Wanted to repeat the operation on cylinder 6 as this one was making noise some months later.

(1) Notably on extraction of cyl 6 injector no copper seal was found...mystery as to where it could have gone..but carried on and cleaned injector hole thoroughly and fitted a lovely new copper seal.
(2) When refitting wiring loom I inadvertently plugged the cyl 5 injector plug onto the high pressure sensor and the high pressure sensor onto the cyl 5 injector. Doh
Car did not start ofc. When I realized what I had done I swapped the plugs back over but engine would not start. Assuming High Pressure Sensor failure, as it normally takes 5v and the injector plug delivers 12v,
(3) I replaced the high pressure sensor with a shiny brand new one. Car still did not start.

I could see a reasonably long air bubble above the High Pressure Fuel Pump so thought air could be the problem and bought a 1 litre can of Liquid Moly Diesel Purge and some hoses and a small rubber hand pump bulb.

(4) Pumped Liquid Moly Diesel Purge through a small filter, which i purchased new also , through the fuel delivery pipework and out of the fuel return line from engine, through another small filter, back into the pot of of the Liquid Moly Diesel Purge supply. Total of approx 200ml out of the whole 1000ml was consumed somewhere by fuel delivery system (not into tank as this was isolated). I cannot account for where this 200ml went and it certainly did not remove the bubble above the HP Fuel Pump. So still could not get the air out of the system. Car was struggling to turn over at this point

(5) Tried easy start into the air intake after opening the air box.
(6) engine seemed to partially lock ...or was it the battery failing...I could not tell.. no nasty noises just hardly any action and hard struggle to even rotate the engine

Put battery on charge and Went to work for a week and returned to the problem a week later.

(7) Removed and took injector from cyl 6 to diesel system specialist for check, they showed me it was not squirting despite 380 bar applied to it. They suggested it sometimes happens when you remove an injector that it ceases to work as the Peizo types are sensitive. Purchased brand new Bosch Injector £222 and fitted it on cyl 6. Still the engine would not start

(8) I noticed that there appeared to be very little compression when turning the engine over and it seems to turn too freely. Not yet comp tested in any way. How would this happen?

Now I am worried something else might have crept into the problem list and wonder if you guys have any ideas?
Regards OB