I'm having a very similar issue with my 2012 Golf Sportwagen A6. Car has 292,000 kms on it.
I bought it a year ago, and it would randomly take a long time to start, for the first start of the day. Didn't matter if it was +20 or - 20. Car always started. Maybe 1 in 20 times would require extended cranking.
This spring it got a lot worse. Now it's 1 in 2 first starts of the day.
I always let the glow plugs cycle. The car will spin and spin and spin. I stop, key off, cycle plugs and try again. Sometimes I need to do this a third time, car will start, run rough for a couple seconds, then works perfect. I have tried just spinning it over until it starts, eventually the car starts flashing low oil pressure on the dash. I don't think it's fuel draining back because I left the car parked for 6 weeks this summer while I drove my other car, and when I got back to it, the battery didn't have much juice, but it still started up right away after a few slow cranks. Car has NOT had the fix, and the condition of the DPF is unknown - car seems to run good, and gets acceptable fuel milage.
Thoughts?