I think 10k miles is OK if you're running 229.5 spec 5W40 and with your driving style.
The concern regarding the E85 is about it diluting the oil, but if you're not driving short distances in the cold, then it should not be an issue (such as Honda 1.5T driving long vs short distance and the oil dilution issue it faces). You appear to be doing plenty of highway for the gas to evaporate out of the crankcase.
With certs such as 229.5, Mercedes-benz approves that it lubes their engines sufficiently and for how long they spec oil change intervals, ie GM wouldn't have Dexos certification/approve an oil as "dexos approved" and think it wouldn't work fine for the intervals they spec.
Plus you said your engine is clean, so 10k shouldn't be a problem imo.
Source: work at a MB dealer. I have never seen an oil related failure from 10,000 OCIs (also specified for your vehicle). Even the twin turbo DI engines of these days still specify 229.5 at 10,000 intervals, and they are perfectly fine with 10,000 miles oil changes.