Sprstpd: I found this thread today (it's 2017 now), and you are still the only one on the internet who properly explained this. Kudos!
I ran into this with UVXY, and I figured the cumulative adjustments to tax basis would somehow offset the capital gain/loss reported on K-1 (I noticed that they add up to about the same amount as is reported in 11C), but wasn't 100% sure and didn't know exactly how to file it. I still have a question on that: would you file the adjustment entries under the same classification as the original ETF is reported, e.g. my UVXY trades are reported to me as section B (short tem, tax basis not reported to IRS), so the adjustment would be 'B' as well, right?
BTW, I wanted to share some observations that helped me understand the issue. For Proshare ETFs, you can go to
https://www.taxpackagesupport.com/ and access your K-1 online, and you can also edit your transactions. My transactions were originally incomplete, Proshares thought I started the year with zero balance, which was not correct. So I called them and they simply added a new opening transaction and regenerated my K-1. I found that weird, as how can you issue a tax report based on someone giving you a transaction over the phone? Now I understand why: it is eventually irrelevant, because you would offset whatever number is spit out on the K-1. Anyhow, I corrected my starting balance, and my K-1 reported a loss of $90K. I go, WTF, I didn't lose that much! Looking at the transactions, it looked like my broker didn't report a sale on a specific day. So I called Proshares again and added a new transaction. Then my K-1 income jumped to 33K. That was even more concerning as now I would need to pay ~10K tax on it? what? Then I realized that it wasn't a missing transaction, but a reverse split that UVXY had. So I got that corrected again, and now my income is only 6K. And then I realized: this number is bogus. I have multiple version of my K-1 now, and in each case, the gain/loss reported on K-1 is being offset by the cumulative adjustments in the sales schedule. I could pretty much file anything, even wrong data entered with Proshares over the phone, because whatever I report on the K-1 boxes, would be offset by the sales schedule adjustment. Duh. What a stupid and confusing form.