Hello Luke,
Before you tie yourself up in knots
Just take a breather and take a look at your live data and fault codes if you have any, as you said you would do this morning.
From there you will be able to see the rail pressure and pressure drop with Key On Engine Off
and measure leak back.
I would imagine what happens when you prime the filter you are re-filling fuel rail via return pipes, thats why it then starts ok.
Also remember by changing any obvious faulty injector doesn't mean another will then fault due to the increase of substained rail pressure.
HTH,
Ian