Electrical/fuel gremlins

Hi folks. Since getting my 2010 A6 Quatro Avant 2.7 on the road I have had a myriad of electrical issues which I have been slowly getting through, but a couple are really getting me frustrated. Main ones being.

1/ Fuel issues. P0087. Replaced in tank fuel pump, both fuel sensors on the fuel rails and fuel filter, but still getting the glow plug warning on hard acceleration. If I accelerate normally, then no issue, but if I  press hard and she goes to kickdown, off it goes. I have access to a Vidas unit which tells me low fuel pressure.

2/ No horn. Fuses all checked and found to be all good. Nothing coming up on Diagnostic. Bear in mind I have only just found how to do the diagnostic and I am FAR from an expert.

3/ Minor issue. water leaking into the tailgate which as blown offside courtesy light on the tailgate and is badly corroded. Is there a common place where water can get in as I can see nothing.

Other than that I am getting to like the Quatro after having run Volvo XC90's for the past 10 years or so.

Desperate to fix issue No 1 in particular as I have a 600 mile round trip to do towing a car transporter covered trailer trailer which will be about 2.4 tons and don't fancy hitting a steep incline and her dropping into limp mode for obvious reasons.

Cheers in advance.

John.