I know what you mean about dead end forum threads Matt. I recently did a mammoth read of this B7 forum, so others don't have to lol; the 170 kept coming up time and time again with these kind of issues, especially when they'd chipped them; usually only sorted with an EGR and DPF delete. Intermittent issues are always pesky and harder to diagnose than constant failure conditions.
The acrid smoke is very often a classic cat/dpf thing - have you been able to determine the soot levels of your DPF with a code reader? Even if it's not throwing an emissions light it could still be high (and ash which is built up over time, is never burnt off, only soot). When it plays up, do you lose boost too? That what's happened when my EGR failed; no smoke but total loss of boost as if my turbo had blown; it was intermittent too, at first. As I say, these early DPFs are hugely problematic with the PD engine tech and deleting them ultimately is the best solution. Is it not trying to regen when you get this condition and never completeing, I wonder too?
I'm not a believer in deleting the EGR, not because I'm a goody two shoes, but because they unintendly aid warm up of the engine, especially in winter, through hot gasses paassing throufgh the EGR cooler, and that means less wear. They are dirty things imposed upon us by emissions regs but they are easy on these engines to strip off and clean every couple of years and as your intake and EGR mine weren't that bad and they'd never been cleaned, by me.
I remember you having concerns over your tandem pump too; they can produce these kind of conditions. The test there is to change the fuel filter and empty the old diesel into a glass jar, if it's anything but completely clear there could be failed gaskets in the pump but dirty fuel can also produce that. At the service I did recently the filter was clear.
I remember a kid on Youtube who had a 170. He had similar issues but he'd had a cheap map put on. He went down a rabbit hole; gutted the DPF, deleted the EGR; put a new tandem pump on; new MAF sensor, with no change whatsoever - and everyone kept telling him your cheap map is the problem. But I don't think yours is chipped in any way?
Anyway, try the ASV disconnect test, run it when you have the MOT for a few miles and see what you get. OBDEleven I believe can test for DPF soot levels as VCDS btw too. It will be something or nothing on such a low mileage good car.