O-ring on the electrical connector is not super critical. It doesn't seal anything in the tank.
It's mainly there to seal the connector from the world around the tank (tank just hangs below the car, and can be subject to water spray, road grime etc).
I don't like it in the video (in vicinity of 10:30) when he says to just pull the pump straight out of the tank.
NO YOU DON'T!
I'll keep saying this:
"it corkscrews out"!
Pulling it straight out puts unnecessary strain on and can damage the hoses.
I made a crack on the main high pressure hose (from the pump to the top of the housing) worse on mine by initially following that advice. On the third pull out of the tank (initially thought I hadn't seated it properly), I had my technique sorted and also found the problem!
You don't need your own marks on the housing either, there's a small triangle at about 4 or 5 o'clock on the housing top that lines up with another on on the tank top.
The big plastic ring nut that holds it all in and seals the tank gets tightened "tight". Mark it of you want. I didn't.
I'll also confirm Pauls advice (of mine) that you don't overtighten the long bolt on installation.
The nut is captive in a weakish plastic housing on the bottom of the in-tank receiver under where the metal-rubber gasket mates, and its very easy to overtighten and end up with it spinning in the housing. Not easy or cheap to solve, so don't do it!
I'm sure Audi has a torque spec for this, but if you have good feel when tightening, you can tighten the long bolt by hand until the rubber starts to bind/compress on the metal rubber gasket. At this point put your spanner on it and gently tighten until the "squishyness" of the rubber becomes solid hard. Stop there - you have now squashed the rubber flat onto the metal part of the gasket, and that is as good a seal as you are going to get. Any tighter risks disaster, and won't achieve anything.
