35d - Long and Multiple Cranks to Start

[Ngkx535d]

So....
Went to the store for groceries and once I got in my vehicle to leave - it wouldn’t start.
Vehicle cranks.....and cranks...and....cranks - but no start.
I was getting all kids of different warnings (most likely due to battery being drained) and decided to call it a night.
Following day - vehicle cranked for about 7 seconds and finally turned over. Drove it home and pulled the code:

4B90 - rail pressure at engine start-to-low

2011 BMW X5 35d ~80k

After a long search, and not finding anything that looks promising, I decided to take it to an Indy for drop off; figured 90 bucks is well spent if I can get a definite answer as to what is causing this.

BMW Master tech - 15 years experience (not sure how much of that time is spend with diesel) had the vehicle for 3 days and couldn’t give me a direct answer.

Low pressure pump - checks good.
High pressure pump - checks good
Injectors - checks good
EKP - checks good

Tech says he uses the same ‘online’ diagnostic system that is live fed to BMW that every Certified BMW service center uses. He stated that all the pressures are good and IF the EKP and/or injectors were bad, he’d definitely be getting other codes.

He suggested that we start with the cheapest course of action - replace fuel filter and drain tank.

I opted to drain the tank (500 bucks) and decided to replace the filter only. Same problem persists.

Sometimes it cranks instantly (cold or hot) but most of the time it take 2-4 long 7 seconds crank to turn over. I am at a lost and I do not want to throw money at my car hoping it will fix it.

Any suggestions?
He did however mention rodent damage at some insulation and a few bite marks on couple of wires under the hood. He couldn’t confirm if that’s the cause but he wanted me to take note of it.

Suggestions/ideas/similar experiences??

Thanks!