Our engines naturally produce a lot of soot, especially when they're driven lightly. The ISV is a very powerful engine, far more than our trucks really need. So for most of the time, the ecm is busy trying to find ways to elevate the exhaust temperature to meet emissions, this is usually accomplished with maintaining a fairly high drive pressure ratio.
The DEF injector is after the dpf, so the failure of the def system likely triggered fallback emissions which stressed out the dpf, while also inhibiting the regen process.
Another issue is that it's possible the catalyst is too clogged to do its job. The catalyst part of the DPF lowers the combustion temperature of the raw diesel fuel in the exhaust, if it's clogged it won't get up to temperature and thus won't be able to burn off the trapped soot.
If warranty doesn't help, I'd look at buying up a used dpf from someone else who has deleted... or just throw in the towel and delete. If you're really brave, you could remove the dpf and try to clean the catalyst (the first visible part of the dpf on the inlet side). The catalyst is made with rare earth metals so try to avoid severe mechanical means of cleaning. Compressed air might get enough soot out to expose the catalyst again.