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100% on the walnut blasting.
When you do cold driving, you are in the optimal conditions to build up carbon on the intake valves. This isn't just a BMW problem, this is a direct injection problem in general. My previous platform (2.3 DISI Mazdaspeed) had the same issues. We all delete the exhaust gas recirculation and install an oil catch can. This helps slow down the build up on the intake valves. The only way to keep them clean really is to have a port injection or water/meth injection setup and you get into high enough boost to run these systems on a consistent basis...or just clean the valves every 30-40k. When you run a condition like you, this needs to be done more often.
I'd also just go ahead and replace your sparks as well.
It should ALSO be noted that the condition you run dilutes the oil with fuel; I would change my oil more often. The most wear to an engine occurs within the first few minutes of cold starts due to its viscosity/how well the oil flows.
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