It is probably a bit naughty but I allow my bulb foliage 6 weeks of good sunlight after flowering or emerging, in the case of colchicums, then chop them back. Sometimes they are still green. I am sure this is not the optimum treatment (and no doubt someone will jump in and give me a metaphorical slap)but in a tiny garden (36 square metres), I am not going to be looking at yellowing slimy leaves a moment longer than I have to. They always return, too.

I think 6 weeks is plenty Campanula. You'll get no flak from me on that one. I always presume that once the leaves begin to yellow they are no longer doing the bulb much good so they can be cut at that point. FYI, the Colorblends catalog describes the Dutch bulb growing process and it says they let the foliage grow for 6 weeks - so you are right on schedule!

I also think that cutting the leaves back somewhat to make them look less shabby is fair game. No need to have the garden look poor, if you ask me!