After riding Cannondales since 1993, (that is not a typo), I have moved on to a different brand, but still have several Cannondales, including a Scalpel with an open recall, that I found out is preventing me from selling it. All the dealers in the Front Range of Colorado (I think I may have called all of them) tell me that Cannondale doesn’t return phone calls regarding service or warranties. I dropped off the bike at an REI, because they said they would try, and are probably the largest dealer in the US. They cannot get an answer, and said they will keep trying. Does anyone on the forum have a contact or recommendation? Thanks,
My one experience with a Cannondale warranty claim was positive. The paint had started to develop hairline cracks in a few places on a bike I had owned for nine months. It was a cosmetic defect and wasn’t super noticeable, so I didn’t necessarily expect them to do anything. But Cannondale replaced the frame and fork paid for the bike shop’s labor to build it up. The whole process took about two weeks. The bike shop said this was typical for Cannondale in their experience. This was in January 2020.
Sorry, I can’t suggest anything beyond what you have tried, but apparently they are responsive in some cases, or at least they have been 
.
In the UK I’m afraid I’ve only got positive experiences as well, likewise a frame warranty for a cosmetic crack. Does seem very unusual that dealers can’t get hold of them.
No tips in the US?