Unless the DV O-rings were just pouring fuel on top of the IP, it's unlikely to be your air source. The O-rings on the DV holders are a secondary seal.

You already bypassed the most common source of air ingress - the fuel heater thermostat.

Other common sources of air leaks are the crush washers on the fuel filter housing, the O-rings on the nut on the fuel filter housing, the flare nut on the lift pump, the O-rings in the lift pump itself (I wrote a whole thread on my saga with said pump), the rubber fuel lines in the engine bay, and the clamps holding said rubber fuel lines.

A lot of folks swear by 5/16" fuel line in the engine bay. I guess I'm one of the minority that absolutely could not get it to seal adequately on my car, no matter what I did. I ordered the correct 7mm fuel line and haven't had an issue since. Another thing that makes a difference is the clamp you use - make sure you use fuel injection hose clamps. Worm drive clamps WILL leak air (ask me how I know!).

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]