To avoid diacetyl, and buy premade flavored juices, you need to make sure the vendoor does not use FlavorArt flavorings. They are from Italy and are the only flavor co I know of that still uses diacetyl, although they just recently eliminated it too. That will not change whatever flavors the vendors have now, however. All US flavors are diacetyl-free: Lorann, Capella, Perfumers Apprentice, etc. And of course, the only way to know for sure what is or is not in your juice is to DIY from scratch.
This all said, it is not likely your coffee juice has diacetyl, unless it is a dessert-style coffee, since diacetyl is butter flavor. The clogging is common for dark juices, such as coffees or tobaccos. I don't get clogging with the chocolates I have tried, however, so I think it is not just a dark thing, but rather a particulate thing, and tobaccos especially are notorious for this.
Another thing that might be causing clogging is if you are using weak batteries, since your juices do have 50% VG. Regular 510 batts are around 3.1V working, and I found that attys tend to gunk up faster on them then with higher voltage, since they run hotter then and tend to vaporize the juice more. You seem to imply this is only a coffee thing, though, so this might not be applicable. Details about the PV in the future might shed more light on these sorts of issues though. Some of the higher resistance atty types on regular batts really need a thin juice to stay healthy.
